ISO/PRF 22785-8

• Abstract:

  This document provides information for the determination of the film thickness of coatings on plastics, plastic composites and similar substrates. An overview of common test methods for determination of wet-film thickness and dry-film thickness is provided.

• Status: Not Published
• ICS: 83.080.01 - Plastics in general
• ICS: 83.120 - Reinforced plastics
• ICS: 87.040 - Paints and varnishes
• Technical Committee: ISO/TC 35/SC 9 - General test methods for paints and varnishes
• Drafting Committee: ISO/TC 35/SC 9/WG 31 - Properties after application
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 01-Oct-2026
• Completion Date: 10-Oct-2026

ISO/PRF 22785-8 is an ISO document on measuring the film thickness of coatings on plastics, plastic composites and similar substrates. It covers both wet-film thickness and dry-film thickness, and it gives an overview of the common methods used for each. For engineers, quality managers and buyers, it is a guide to which measurement principle fits the substrate, coating state and test purpose.

What does ISO/PRF 22785-8 specify?

ISO/PRF 22785-8 specifies how to determine the film thickness of coatings on plastics, plastic composites and similar substrates. It also organizes the subject into the specimen requirements, wet-film thickness methods, and dry-film thickness methods.

The document is organized into:

• Scope
• Normative references
• Terms and definitions
• Test specimen characteristics and preparation
• Determination of wet-film thickness
• Determination of dry-film thickness
• Bibliography

The introduction explains why the subject is different from metal substrates: plastics are generally not electrically conductive and not magnetic, so magnetic and electromagnetic thickness methods used on metals are not suitable here. In practice, that tells the reader to choose a method designed for plastic substrates rather than reusing a metal-coating method.

What are the key requirements of ISO/PRF 22785-8?

ISO/PRF 22785-8 focuses on selecting a suitable method, preparing the specimen correctly, and understanding the limits of each measurement principle. The most useful requirements are in Clauses 4, 5 and 6.

Specimens and conditioning matter first

Clause 4 says coatings may be tested on standard panels as specified in ISO 1514 or on actual components. The chosen method can impose different geometry requirements, so the shape of the part matters before testing starts.

Clause 4.2 also stresses conditioning. Plastics are more sensitive to humidity than metals, so conditioning often takes longer, and the values and times depend on the specimen. In practice, this means results are only comparable when the specimen state is controlled before measurement.

Wet-film thickness can be measured by several principles

Clause 5 describes mechanical, gravimetric and photothermal methods for wet-film thickness.

• Mechanical methods use a comb gauge, wheel gauge or dial gauge and are intended for direct reading immediately after application. The document notes that the coating can start drying, curing or losing solvent quickly, so timing matters.
• The mechanical principle is suitable for all film-substrate combinations if the surface is flat in at least one direction. A curvature in one plane is allowed, such as on pipes.
• The text also warns that all mechanical wet-film methods can show a systematic under-reading because pigment particles may remain between the gauge and the substrate. For a wheel gauge, incomplete wetting of the wheel can create additional error.

For users, this means the gauge choice is not only about convenience - it also affects the accuracy limits and the risk of bias.

Wet-film comb and wheel gauges have practical limits

Clause 5.1.4 describes the wet film comb. It is placed on the surface, allowed to wet, and the greatest wetted gap is taken as the wet-film thickness. Commercial comb gauges are typically used up to 2 000 µm with 5 µm increments.

Clause 5.1.5 describes the wheel gauge. The wheel is rolled in both directions, cleaned between readings, and the mean of the readings is used. Version 2 allows parallax-free reading, which matters when the operator wants to reduce reading error.

Wet-film gravimetry gives a mean over the whole area

Clause 5.2 determines wet-film thickness from specimen mass, coated area and liquid coating density. The method is generally applicable if the liquid coating has little volatile content. It gives the mean wet-film thickness over the entire coated area, so it is useful when a global average is needed rather than a local spot value.

The text also notes that overspray on the reverse side can distort results, so masking may be needed on sprayed specimens. In practice, this is a weighing method, so density data from the referenced ISO 2811 series is part of the measurement chain.

Wet-film photothermal methods need calibration

Clause 5.3 uses thermal excitation and detection to infer thickness from the response time of the coating. It is suitable for all film-substrate combinations and can also be used on multilayer coatings if the layers differ enough in thermal conductivity and reflection properties.

The measurement system is calibrated with reference materials for each film-substrate combination, especially for each coating material. That means the method is not just an instrument setting - the calibration set has to match the material pair being tested.

Dry-film thickness can be measured directly or by difference

Clause 6 covers dry-film thickness by mechanical, gravimetric, optical, photothermal, acoustic and terahertz methods.

• Mechanical methods compare substrate thickness and total thickness, either before and after coating removal or before and after coating application.
• The coating-application variant is preferred for plastics because exposing the substrate can damage it.
• The profilometer is stated as the referee method in disputes, which makes it important when parties need a tie-break method.
• The dial gauge, dial depth gauge and profilometer all depend on clean surfaces and careful specimen preparation.

For a buyer or lab, this means the document distinguishes between routine methods and methods used when results may be challenged.

Dry-film gravimetric, optical, acoustic and terahertz methods support different test cases

Clause 6.2 gives the gravimetric method, which again yields a mean over the whole coated surface and uses dry-film density from the ISO 3233 series. Clause 6.3 gives optical methods, including cross-grinding/cross-sectioning and the wedge cut method, and also a beta backscatter method with verification against reference standards.

Clause 6.5 describes an acoustic method based on ultrasonic travel times, with sensitivity to sound-speed uniformity and to inhomogeneities in the coating or substrate. Clause 6.6 describes the terahertz method for multilayer systems, especially when the individual layers have sufficiently different refractive index and extinction coefficient. In practice, these methods are most useful when the coating structure, material contrast or test dispute calls for more than a simple mechanical reading.

What terms does ISO/PRF 22785-8 define?

ISO/PRF 22785-8 uses a small set of core terms to keep thickness measurements consistent.

• Film thickness - the distance between the surface of the film and the surface of the substrate.
• Wet-film thickness - the thickness of a freshly applied wet coating, measured immediately after application.
• Dry-film thickness - the thickness of the coating that remains after the coating has hardened.
• Test area - a representative part of the surface where a defined number of single measurements is made as a spot-check.

These terms matter because the document uses different methods, timing and calculations depending on whether the coating is wet or dry.

Who uses ISO/PRF 22785-8?

ISO/PRF 22785-8 is used by coating laboratories, paint and varnish technologists, production quality teams, and people checking coated plastic parts in manufacturing or supply chains. It is also relevant to buyers and quality managers who need to specify how coating thickness should be measured on plastic substrates.

The document is useful when a team must choose between local spot measurements and whole-area mean values, or when it needs a method that fits a component shape, a coating state, or a dispute about measurement results.

Which standards are used with ISO/PRF 22785-8?

Standard	What it contributes
ISO 4618	Vocabulary for paints and varnishes used in the terminology of this document.
ISO 22785-1	The general introduction to the ISO 22785 series.
ISO 1514	Standard panels for testing, used for test specimens in Clause 4.
ISO 463	Design and metrological characteristics of mechanical dial gauges used in thickness measurement.
ISO 2811-1 to ISO 2811-4	Density of liquid coating material for gravimetric wet-film thickness calculations.
ISO 3233-1 to ISO 3233-3	Dry-film density for gravimetric dry-film thickness calculations.
ISO 19397	Ultrasonic gauge principles for the acoustic method.
EN 15042-2	Guidance for photothermal thickness measurement and reference materials.

What does the ISO/PRF 22785-8 document contain?

ISO/PRF 22785-8 contains method descriptions, principles of operation, field-of-application notes, and step-by-step procedures for wet-film and dry-film thickness measurement. It also includes formulas for gravimetric calculation and for wedge-cut thickness calculation.

The body of the document gives instrument descriptions and figures for the comb gauge, wheel gauge, dial gauges, depth gauge, profilometer, beta backscatter setup, acoustic probe arrangement, and terahertz measurement principle. Those figures help the reader understand how the measurement geometry works before choosing a method.

The document also explains calibration or verification needs for photothermal, beta backscatter, acoustic and terahertz methods. For practical use, that means the standard is not only about taking a reading - it also tells the user what reference materials and setup conditions are part of a defensible measurement.

International
Standard
ISO 22785-8
First edition
Paints and varnishes — Coatings on
plastics and composites —
Part 8:
Film thickness
PROOF/ÉPREUVE
Reference number
ISO 22785-8:2026(en) © ISO 2026

ISO 22785-8:2026(en)
© ISO 2026
All rights reserved.
This ISO publication is protected by copyright and is owned by ISO and/or its licensors.
The content of this ISO publication is provided under licence, not sold. Use is subject to the applicable licence terms issued by ISO,
an ISO member body, or an authorized third-party distributor.
Except as required for implementation or expressly permitted by a separate licence, no part of this ISO publication may be
reproduced, distributed, modified, used, or made available in any form or by any means – electronic or mechanical, including
photocopying, scanning, recording, or posting on internal or external digital platforms.
Any use beyond the scope of the granted rights is prohibited and may result in legal action.
ISO copyright office
CP 401 • CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
PROOF/ÉPREUVE
ii
ISO 22785-8:2026(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test specimen characteristics and preparation . 2
4.1 Test specimens.2
4.2 Conditioning.2
5 Determination of wet-film thickness . 2
5.1 Mechanical methods .2
5.1.1 Principle .2
5.1.2 Field of application .2
5.1.3 General .2
5.1.4 Wet film comb .3
5.1.5 Wheel gauge .4
5.1.6 Dial gauge.5
5.2 Gravimetric method .6
5.2.1 Principle .6
5.2.2 Field of application .7
5.2.3 General .7
5.2.4 Procedure .7
5.3 Photothermal method .7
5.3.1 Principle .7
5.3.2 Field of application .8
5.3.3 General .8
5.3.4 Instrument and reference materials (reference samples) .8
5.3.5 Calibration . . .8
5.3.6 Procedure .9
6 Determination of dry-film thickness . 9
6.1 Mechanical methods .9
6.1.1 Principle .9
6.1.2 Filed of application .9
6.1.3 General .9
6.1.4 Dial gauge.10
6.1.5 Dial depth gauge .11
6.1.6 Profilometer . 12
6.2 Gravimetric method . 13
6.2.1 Principle . 13
6.2.2 Field of application .14
6.2.3 General .14
6.2.4 Procedure .14
6.3 Optical method .14
6.3.1 Principle .14
6.3.2 Cross-grinding/cross-sectioning . 15
6.3.3 Wedge cut .16
6.3.4 Beta backscatter method .17
6.4 Photothermal method .17
6.4.1 Principle .17
6.4.2 Field of application .18
6.4.3 General .18
6.4.4 Apparatus and reference materials (reference samples) .18
6.4.5 Verification .19
6.4.6 Procedure .19
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ISO 22785-8:2026(en)
6.5 Acoustic method . .19
6.5.1 Principle .19
6.5.2 Field of application .19
6.5.3 General .19
6.5.4 Description of the instrument .19
6.5.5 Procedure . 20
6.6 Terahertz method . 20
6.6.1 Principle . 20
6.6.2 Field of application .21
6.6.3 Description of instrument.21
6.6.4 Procedure . 22
Bibliography .23
PROOF/ÉPREUVE
iv
ISO 22785-8:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee SC 9,
General test methods for paints and varnishes.
A list of all parts in the ISO 22785 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
PROOF/ÉPREUVE
v
ISO 22785-8:2026(en)
Introduction
The chemical and physical properties of plastics and composites as substrates are very different from those
of metallic substrates.
In contrast to metals, plastics generally are not electrically conductive and are not magnetic. For this reason,
magnetic and electromagnetic methods cannot be used to measure film thickness on plastic substrates.
PROOF/ÉPREUVE
vi
International Standard ISO 22785-8:2026(en)
Paints and varnishes — Coatings on plastics and
composites —
Part 8:
Film thickness
1 Scope
This document specifies the determination of the film thickness of coatings on plastics, plastic composites
and similar substrates.
It also provides an overview of common test methods for determination of wet-film thickness and dry-film
thickness.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 4618, Paints and varnishes — Vocabulary
ISO 22785-1, Coatings on plastics and composites — Part 1: General introduction
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4618 and ISO 22785-1 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
film thickness
distance between the surface of the film and the surface of the substrate
[SOURCE: ISO 2808:2019, 3.1]
3.2
wet-film thickness
thickness of a freshly applied wet coating material, measured immediately after application
[SOURCE: ISO 2808:2019, 3.2]
3.3
dry-film thickness
thickness of a coating remaining on the surface when the coating has hardened
[SOURCE: ISO 2808:2019, 3.3]
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ISO 22785-8:2026(en)
3.4
test area
representative part of the relevant surface area, within which an agreed number of single measurements is
made as a spot-check
[SOURCE: ISO 2808:2019, 3.6, modified — Note 1 to entry has been deleted.]
4 Test specimen characteristics and preparation
4.1 Test specimens
Coatings both on test specimen panels as specified in ISO 1514 and on components are tested. Depending on
the selected test method, there are different requirements for the geometry of the test specimen.
Any type of flaw or damage impairs the appearance and properties of the substrate.
The coating system and the substrate are selected together with the pre-treatment and coating methods so
that a homogeneous coating is achieved. The film thickness is documented in accordance with the technical
requirements relating to the visual appearance and/or coating.
4.2 Conditioning
Conditioning serves to improve the repeatability and reproducibility of test results.
As the properties of plastics substrates depend greatly on humidity levels and conditioning usually takes
longer than for metals, a longer conditioning time is usually required for plastics substrates.
The values and times to be selected depend on the test specimen to be tested.
5 Determination of wet-film thickness
5.1 Mechanical methods
5.1.1 Principle
With mechanical procedures, the measuring instrument (see Figures 1, 2 and 3, Key 4) passes through by the
coating being placed on the surface of the substrate. The difference between the points of contact (substrate,
see Figures 1, 2 and 3, Key 1) and the coating surface touched from the top of the measurement instrument
(Figures 1, 2 and 3, Key 3) corresponds to the readable wet-film thickness.
5.1.2 Field of application
The mechanical principle is suitable for all film-substrate combinations. The substrate is flat in at least one
direction in the area where the measurement is performed. Curvature of the surface in a single plane is
permissible (e.g. internal or external surface of pipes).
5.1.3 General
The classification as a destructive or non-destructive process depends on
a) the rheological properties of the coating material;
b) the nature of the wetting contact between the contact surfaces of the measurement instrument and the
coating material;
c) whether the thickness measurements will make the coating unsuitable for the purpose for which it is
intended.
PROOF/ÉPREUVE
ISO 22785-8:2026(en)
Since the possibility of pigment particles remaining between the gauge and the substrate cannot be excluded,
all mechanical methods contain a systematic error: the film thickness displayed is smaller than the actual
wet-film thickness by at least the mean diameter of the pigment particles.
In the case of a wheel gauge (5.1.5), the wheel is wetted by the coating material. If not, this represents a
further source of systematic error which can result in exaggerated readings and is a function of:
— the surface tension and the rheological properties of the coating material;
— the material of the wheel gauge;
— the speed at which the wheel is turned.
5.1.4 Wet film comb
5.1.4.1 Description of instrument
A comb gauge is a flat plate made of a corrosion-resistant material with teeth along its edges (see Figure 1).
The reference teeth at the corners of the plate specifies a baseline along which the inner teeth are arranged
to give a graduated series of gaps. Each tooth is labelled with the assigned gap value.
With commercially available comb gauges, the maximum thickness which can be measured is typically
2 000 µm and the smallest increment is typically 5 µm.
Dimensions in micrometres
Key
1 substrate
2 coating
3 point of wetting contact
4 comb gauge
Figure 1 — Example of a comb gauge
5.1.4.2 Procedure
As the first step, it is ensured that the teeth are clean and not worn out or damaged. The comb gauge is
placed onto the flat specimen surface such that the teeth are normal to the plane of the surface. Sufficient
time is allowed for the coating to wet the teeth prior to removing the gauge.
In the case of specimens curved in a single plane, the comb gauge is placed in position parallel to the axis of
curvature.
The thickness is measured directly after application since the result can be influenced, for example, by
physical drying, curing or solvent loss.
PROOF/ÉPREUVE
ISO 22785-8:2026(en)
The greatest gap reading of the tooth wetted by the coating material is taken as the wet-film thickness.
5.1.5 Wheel gauge
5.1.5.1 Description of instrument
A wheel gauge consists of a wheel, made of hardened and corrosion-resistant steel, with three projecting
rims (see Figure 2).
Two rims are ground to the same diameter and are configured concentrically to the wheel axle. The third
rim has a smaller diameter and is eccentrically ground. One of the outer rims has a scale from which the
respective protrusion of the concentric rims relative to the eccentric rim can be read.
Two versions are available:
— version 1 has the eccentric rim located between the concentric rims;
— version 2 has the eccentric rim located systematically outside the concentric rims and closely adjacent to
one of them.
Unlike version 1, the design of version 2 allows parallax-free reading of the wet-film thickness.
With commercially available wheel gauges, the maximum thickness which can be measured is typically
1 500 µm and the smallest increment is typically 2 µm.
Key
1 substrate
2 coating
3 eccentric wheel
4 wheel gauge
Figure 2 — Wheel gauge
5.1.5.2 Procedure
The wheel gauge is griped with the thumb and index finger by the wheel axle and the concentric rims are
pressed onto the surface at the point of the largest reading on the scale.
In the case of specimens curved in a single plane, the axis of curvature and the wheel gauge axle are parallel.
The wheel gauge is rolled in one direction, lifted from the surface and the highest scale reading at which the
eccentric rim is still wetted by the coating material is read off. The gauge is cleaned and the measurement is
repeated in the other direction.
PROOF/ÉPREUVE
ISO 22785-8:2026(en)
The wet-film thickness is calculated as the arithmetic mean of these readings.
The thickness is measured directly after application since the result can be influenced, for example, by
physical drying, curing or solvent loss.
To minimize the influence of surface tension on the measurement result, the wetting process is observed
directly, and the scale reading is taken at the moment of initial wetting. This is only possible with version 2
of the wheel gauge.
5.1.6 Dial gauge
5.1.6.1 Instrument and reference material (reference standard)
Analogue dial gauges conforming to the requirements of ISO 463 [see Figure 3 a)] are typically capable of
measuring to an accuracy of 5 µm and digital dial gauges [see Figure 3 b)] to an accuracy of 1 µm, or greater.
The gauge can have an analogue or digital display.
The underside of the dial gauge has two contact pins of equal length located equidistant from the movable
plunger and in a straight line with it. An adjusting screw is used to make careful adjustments to the position
of the plunger in its guideway.
a) Analogue dial gauge
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ISO 22785-8:2026(en)
b) Digital dial gauge
Key
1 substrate
2 coating
3 plunger
Figure 3 — Examples of an analogue dial gauge and a digital dial gauge
A flat reference plate is required for zeroing the gauge. The reference plate shall be a polished glass plate.
5.1.6.2 Procedure
The dial gauge is zeroed on the reference plate with the measuring tip adjusted so that it is just touching the
plate.
The plunger is screwed back from the zeroing position.
The contact pins of the dial gauge are placed on the specimen so that they are perpendicular to the surface of
the substrate and the plunger is carefully lowered down until the measuring tip is just touching the coating
material.
In the case of specimens curved in a single plane, the plungers are placed in position parallel to the axis of
curvature.
The thickness is measured directly after application since the result can be influenced, for example, by
physical drying, curing or solvent loss.
The wet-film thickness is read directly from the gauge.
5.2 Gravimetric method
5.2.1 Principle
A coating is applied and the thickness is determined by dividing the mass of the coating by its density and by
the coated surface area.
The wet-film thickness, t , in micrometres, is calculated with Formula (1):
w
PROOF/ÉPREUVE
ISO 22785-8:2026(en)
mm
t  (1)
w
A
where
m is the mass of the uncoated test specimen, in grams;
m is the mass of the coated test specimen, in grams;
A is the coated surface, in square metres;
ρ is the density of the liquid coating material applied, in grams per cubic centimetres.
NOTE 1 The density of the liquid coating material applied can be determined according to ISO 2811-1, ISO 2811-2,
ISO 2811-3 or ISO 2811-4.
NOTE 2 For coatings on plastics substrates, the gravimetric method is less precise than the other methods
described in this document.
5.2.2 Field of application
The gravimetric measuring principle is generally applicable, provided the liquid coating material contains
no or only
...

ISO/DISPRF 22785-8:2026(en)
ISO/TC 35/SC 9
Secretariat: BSI
Date: 2026-05-2209-30
Paints and varnishes — Coatings on plastics and composites —
Part 8:
Film thickness
© PROOF
ISO/DISPRF 22785-8:2026(en)
All rights reserved. Unless otherwise specified,
This ISO publication is protected by copyright and is owned by ISO and/or its licensors.
The content of this ISO publication is provided under licence, not sold. Use is subject to the applicable licence terms issued
by ISO, an ISO member body, or an authorized third-party distributor.
Except as required in the context of its for implementation or expressly permitted by a separate licence, no part of this
ISO publication may be reproduced or utilized otherwise, distributed, modified, used, or made available in any form or
by any means, – electronic or mechanical, including photocopying, scanning, recording, or posting on internal or external
digital platforms.
Any use beyond the internet or an intranet, without prior written permission. Permission can be requested from either
ISO at the address below or ISO’s member body in the countryscope of the requestergranted rights is prohibited and may
result in legal action.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
iii
ISO #####-#:####(X/PRF 22785-8:2026(en)
Contents
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test specimen characteristics and preparation . 2
4.1 Test specimens . 2
4.2 Conditioning . 2
5 Determination of wet-film thickness . 2
5.1 Mechanical methods . 2
5.2 Gravimetric method . 9
5.3 Photothermal method . 9
6 Determination of dry-film thickness . 11
6.1 Mechanical methods . 11
6.2 Gravimetric method . 18
6.3 Optical method . 18
6.4 Photothermal method . 23
6.5 Acoustic method . 24
6.6 Terahertz method . 26
Bibliography . 29

© ISO #### 2026 – All rights reserved
iv
ISO/DISPRF 22785-8:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee SC 9,
General test methods for paints and varnishes.
A list of all parts in the ISO 22785 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
ISO #####-#:####(X/PRF 22785-8:2026(en)
Introduction
The chemical and physical properties of plastics and composites as substrates are very different from those of
metallic substrates.
In contrast to metals, plastics generally are not electrically conductive and are not magnetic. For this reason,
magnetic and electromagnetic methods cannot be used to measure film thickness on plastic substrates.
© ISO #### 2026 – All rights reserved
vi
DRAFT International Standard ISO/DIS 22785-8:2026(en)

Paints and varnishes — Coatings on plastics and composites —
Part 8:
Film thickness
1 Scope
This document specifies the determination of the film thickness of coatings on plastics, plastic composites and
similar substrates.
It also provides an overview of common test methods for determination of wet-film thickness and dry-film
thickness.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 4618, Paints and varnishes — Vocabulary
ISO 22785--1, Coatings on plastics and composites — Part 1: General introduction
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4618, and ISO 22785-1 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 3.1
film thickness
distance between the surface of the film and the surface of the substrate
[SOURCE: ISO 2808:2019, 3.1]
3.2 3.2
wet-film thickness
thickness of a freshly applied wet coating material, measured immediately after application
[SOURCE: ISO 2808:2019, 3.2]
3.3 3.3
dry-film thickness
thickness of a coating remaining on the surface when the coating has hardened
[SOURCE: ISO 2808:2019, 3.3]
3.4 3.4
test area
representative part of the relevant surface area, within which an agreed number of single measurements is
made as a spot-check
[SOURCE: ISO 2808:2019, 3.6, modified — Note 1 to entry has been deleted].]
4 Test specimen characteristics and preparation
4.1 Test specimens
Coatings both on test specimen panels as specified in ISO 1514 and on components are tested. Depending on
the selected test method, there are different requirements for the geometry of the test specimen.
Any type of flaw or damage impairs the appearance and properties of the substrate.
The coating system and the substrate are selected together with the pre-treatment and coating methods so
that a homogeneous coating is achieved. The film thickness is documented in accordance with the technical
requirements relating to the visual appearance and/or coating.
4.2 Conditioning
Conditioning serves to improve the repeatability and reproducibility of test results.
As the properties of plastics substrates depend greatly on humidity levels and conditioning usually takes
longer than for metals, a longer conditioning time is usually required for plastics substrates.
The values and times to be selected depend on the test specimen to be tested.
5 Determination of wet-film thickness
5.1 Mechanical methods
5.1.1 Principle
With mechanical procedures, the measuring instrument (see Figures 1Figures 1, 2, 2 and 33,, Key 4) passes
through by the coating being placed on the surface of the substrate. The difference between the points of
contact (substrate, see Figures 1Figures 1, 2, 2 and 33,, Key 1) and the coating surface touched from the top of
the measurement instrument (Figures 1(Figures 1, 2, 2 and 33,, Key 3) corresponds to the readable wet-film
thickness.
5.1.2 Field of application
The mechanical principle is suitable for all film-substrate combinations. The substrate is flat in at least one
direction in the area where the measurement is performed. Curvature of the surface in a single plane is
permissible (e.g. internal or external surface of pipes).
5.1.3 General
The classification as a destructive or non-destructive process depends on
a) a) the rheological properties of the coating material;
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ISO/DISPRF 22785-8:2026(en)
b) b) the nature of the wetting contact between the contact surfaces of the measurement instrument
and the coating material;
c) c) whether the thickness measurements will make the coating unsuitable for the purpose for
which it is intended.
Since the possibility of pigment particles remaining between the gauge and the substrate cannot be excluded,
all mechanical methods contain a systematic error: the film thickness displayed is smaller than the actual wet-
film thickness by at least the mean diameter of the pigment particles.
In the case of a wheel gauge (5.1.5(5.1.5),), the wheel is wetted by the coating material. If not, this represents
a further source of systematic error which can result in exaggerated readings and is a function of:
— — the surface tension and the rheological properties of the coating material;
— — the material of the wheel gauge;
— — the speed at which the wheel is turned.
5.1.4 Wet film comb
5.1.4.1 Description of instrument
A comb gauge is a flat plate made of a corrosion-resistant material with teeth along its edges (see
Figure 1Figure 1).). The reference teeth at the corners of the plate specifies a baseline along which the inner
teeth are arranged to give a graduated series of gaps. Each tooth is labelled with the assigned gap value.
With commercially available comb gauges, the maximum thickness which can be measured is typically
2 000 µm and the smallest increment is typically 5 µm.
Dimensions in micrometres
Key
1 substrate
2 coating
3 point of wetting contact
4 comb gauge
Figure 1 — Example of a comb gauge
5.1.4.2 Procedure
As the first step, it is ensured that the teeth are clean and not worn out or damaged. The comb gauge is placed
onto the flat specimen surface such that the teeth are normal to the plane of the surface. Sufficient time is
allowed for the coating to wet the teeth prior to removing the gauge.
In the case of specimens curved in a single plane, the comb gauge is placed in position parallel to the axis of
curvature.
The thickness is measured directly after application since the result can be influenced, for example, by physical
drying, curing or solvent loss.
The greatest gap reading of the tooth wetted by the coating material is taken as the wet-film thickness.
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ISO/DISPRF 22785-8:2026(en)
5.1.5 Wheel gauge
5.1.5.1 Description of instrument
A wheel gauge consists of a wheel, made of hardened and corrosion-resistant steel, with three projecting rims
(see Figure 2Figure 2).).
Two rims are ground to the same diameter and are configured concentrically to the wheel axle. The third rim
has a smaller diameter and is eccentrically ground. One of the outer rims has a scale from which the respective
protrusion of the concentric rims relative to the eccentric rim can be read.
Two versions are available:
— — version 1 has the eccentric rim located between the concentric rims;
— — version 2 has the eccentric rim located systematically outside the concentric rims and closely adjacent
to one of them.
Unlike version 1, the design of version 2 allows parallax-free reading of the wet-film thickness.
With commercially available wheel gauges, the maximum thickness which can be measured is typically
1 500 µm and the smallest increment is typically 2 µm.

Key
1 substrate
2 coating
3 eccentric wheel
4 wheel gauge
Figure 2 — Wheel gauge
5.1.5.2 Procedure
The wheel gauge is griped with the thumb and index finger by the wheel axle and the concentric rims are
pressed onto the surface at the point of the largest reading on the scale.
In the case of specimens curved in a single plane, the axis of curvature and the wheel gauge axle are parallel.
The wheel gauge is rolled in one direction, lifted from the surface and the highest scale reading at which the
eccentric rim is still wetted by the coating material is read off. The gauge is cleaned and the measurement is
repeated in the other direction.
The wet-film thickness is calculated as the arithmetic mean of these readings.
The thickness is measured directly after application since the result can be influenced, for example, by physical
drying, curing or solvent loss.
To minimize the influence of surface tension on the measurement result, the wetting process is observed
directly, and the scale reading is taken at the moment of initial wetting. This is only possible with version 2 of
the wheel gauge.
5.1.6 Dial gauge
5.1.6.1 Instrument and reference material (reference standard)
Analogue dial gauges conforming to the requirements of ISO 463 [see Figure 3Figure 3 a)] are typically
capable of measuring to an accuracy of 5 µm and digital dial gauges [see Figure 3Figure 3 b)] to an accuracy of
1 µm, or greater. The gauge can have an analogue or digital display.
The underside of the dial gauge has two contact pins of equal length located equidistant from the movable
plunger and in a straight line with it. An adjusting screw is used to make careful adjustments to the position
of the plunger in its guideway.
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a) Analogue dial gauge
b) Digital dial gauge
Key
1 substrate
2 coating
3 plunger
Figure 3 — Examples of an analogue dial gauge and a digital dial gauge
A flat reference plate is required for zeroing the gauge. The reference plate shall be a polished glass plate.
5.1.6.2 Procedure
The dial gauge is zeroed on the reference plate with the measuring tip adjusted so that it is just touching the
plate.
The plunger is screwed back from the zeroing position.
The contact pins of the dial gauge are placed on the specimen so that they are perpendicular to the surface of
the substrate and the plunger is carefully lowered down until the measuring tip is just touching the coating
material.
In the case of specimens curved in a single plane, the plungers are placed in position parallel to the axis of
curvature.
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The thickness is measured directly after application since the result can be influenced, for example, by physical
drying, curing or solvent loss.
The wet-film thickness is read directly from the gauge.
5.2 Gravimetric method
5.2.1 Principle
A coating is applied and the thickness is determined by dividing the mass of the coating by its density and by
the coated surface area.
The wet-film thickness, t , in micrometres, is calculated with Formula (1)Formula (1):
w
𝑚𝑚−𝑚𝑚
𝑡𝑡 = (1)
w
𝐴𝐴⋅𝜌𝜌
where
m is the mass of the uncoated test specimen, in grams;
m is the mass of the coated test specimen, in grams;
A is the coated surface, in square metres;
ρ is the density of the liquid coating material applied, in grams per cubic centimetres.
NOTE 1 The density of the liquid coating material applied can be determined according to ISO 2811--1, ISO 2811--2,
ISO 2811--3 or ISO 2811--4.
NOTE 2 For coatings on plastics substrates, the gravimetric method is less precise than the other methods described
in this document.
5.2.2 Field of application
The gravimetric measuring principle is generally applicable, provided the liquid coating material contains no
or only a small proportion of volatile components.
5.2.3 General
Determination using the gravimetric principle yields the mean wet-film thickness over the entire coated
surface area. With spray application in particular, the reverse side of the specimen is masked to prevent
measurement errors resulting from partial coating of the reverse (overspray). Any masking of the reverse side
is removed before weighing the coated specimen.
5.2.4 Procedure
The specimen is weighed first uncoated and then coated and the wet-film thickness is calculated using
Formula (1) (1).
5.3 Photothermal method
5.3.1 Principle
The film thickness is determined from the difference between the time a thermal wave is radiated towards the
coating and the time the re-emitted wave (either heat or ultrasonic) is detected (see Figure 4Figure 4).).
Irrespective of the type of excitation involved or the method of detection, all photothermal methods use the
same principle: the periodic or pulsed introduction of energy in the form of heat into a specimen and the
subsequent detection of the local temperature increase.
The time difference measured is compared against values obtained with the instrument for films of known
thickness under fixed conditions (excitation energy, pulse length, excitation frequency, etc.).

Key
1 infrared detector
2 excitation beam
3 thermal radiation
4 test specimen
Figure 4 — Diagram of photothermal detection
5.3.2 Field of application
The photothermal principle is basically suitable for all film-substrate combinations. It can also be used to
determine the thicknesses of the individual layers in a multilayer coating, provided the layers are sufficiently
distinct from each other with respect to their thermal conductivity and reflection properties.
The required minimum substrate thickness is a function of the measurement system used and the film-
substrate combination.
5.3.3 General
The classification as a destructive or non-destructive test method depends on the intended purpose of the
coating. The thermal energy absorbed by the coating could potentially have an influence on the coating
material on account of the localised heating.
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ISO/DISPRF 22785-8:2026(en)
5.3.4 Instrument and reference materials (reference samples)
There are various methods for producing thermal waves in a coating material and for detecting the thermal
effects induced at the heated location in the specimen (see EN 15042--2). Thermal radiation sources (e.g. laser
sources, light-emitting diodes, incandescent light sources) are mainly used as the excitation system for
coatings.
The following detection methods are used:
— — detection of the re-emitted thermal radiation (photothermal radiometry);
— — detection of the change in refractive index (in the heated air above the test area);
— — pyroelectric detection (heat flow measurement).
Reference materials with different absorption properties and a range of film thicknesses are required for
calibration purposes (e.g. see EN 15042--2).
5.3.5 Calibration
The measurement system is calibrated with the reference materials for each film-substrate combination, in
particular for each coating material.
5.3.6 Procedure
The apparatus is operated and the film thickness is measured in according to the
manufacturer'smanufacturer’s instructions.
6 Determination of dry-film thickness
6.1 Mechanical methods
6.1.1 Principle
A dial gauge (see 6.1.46.1.4)) is used to measure the film thickness as the difference between the total
thickness (substrate + film) and the substrate thickness.
There are two ways of determining the film thickness:
a) a) Measurements are made before and after coating removal (destructive).
The total thickness is first measured in a specified test area and then, after the coating has been removed
in the test area, the substrate thickness is measured.
b) b) Measurements are made before and after coating application (non-destructive).
The substrate thickness is first measured and then the total thickness is measured in the same test area
after coating.
The film thickness is calculated from the difference between the two readings.
A dial depth gauge (6.1.5(6.1.5)) or a profilometer (6.1.6(6.1.6)) enable the film thickness to be determined
directly as the difference in height between the surface of the film and the surface of the exposed substrate.
Only the “coating removal” variant is possible with a depth gauge or profilometer.
6.1.2 Filed of application
The mechanical principle is basically suitable for all film-substrate combinations. Mechanical gauging can only
be used if the substrate and coating are constituted so as to prevent the reading from being falsified as a result
of the measuring tip creating an indentation.
The dial gauge is also suitable for measuring the film thickness of c
...