ASTM D5235-18(2024)

Standard Test Method for Microscopic Measurement of Dry Film Thickness of Coatings on Wood Products

SIGNIFICANCE AND USE
5.1 As a base for calibration adjustment or accuracy verification of dry film coating thickness measuring instruments.  
5.2 The dry film thickness of coatings on wood or wood-based products is specified in written product warranties for proper decorative and protective performance of coatings on wood or wood-based products.  
5.3 The minimum and maximum dry film thickness of coatings is recommended by coating companies for satisfactory decorative and protective performance on wood or wood-based products.  
5.4 The average dry film thickness of coatings on wood or wood-based material may be used by manufacturing companies to estimate the theoretical cost of applied coatings.  
5.5 The ratio of minimum to maximum dry film thickness on textured products is used as an indication of coating uniformity.  
5.6 Specific coated product requirements may dictate certain film thickness determinations to be made. Agreement between buyer and seller may be advisable to accommodate product needs relative to dry film thickness.
SCOPE
1.1 This test method covers the measurement of dry film thickness of coatings applied to a smooth, textured or curved rigid substrate of wood or a wood-based product.  
1.2 This test method covers the preparation of wood or wood-based specimens for the purpose of microscopic measurement of dry film thickness.  
1.3 This test method suggests an analysis of dry film thickness of coatings on wood or wood-based products using a microscopic measurement.  
1.4 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Jan-2024
• ICS: 17.040.20 - Properties of surfaces
• ICS: 79.020 - Wood technology processes
• Technical Committee: D01 - Paint and Related Coatings, Materials, and Applications
• Drafting Committee: D01.52 - Factory Coated Wood Products

Relations

• Replaces: ASTM D5235-18 - Standard Test Method for Microscopic Measurement of Dry Film Thickness of Coatings on Wood Products
• Effective Date: 01-Feb-2024
• Referred By: ASTM D2091-96(2024) - Standard Test Method for Print Resistance of Lacquers
• Effective Date: 01-Feb-2024
• Referred By: ASTM D5795-16(2024) - Standard Test Method for Determination of Liquid Water Absorption of Coated Wood and Wood Based Products Via “Cobb Ring” Apparatus
• Effective Date: 01-Feb-2024

Overview

ASTM D5235-18(2024): Standard Test Method for Microscopic Measurement of Dry Film Thickness of Coatings on Wood Products provides a reliable, standardized approach for determining the dry film thickness (DFT) of coatings applied to wood and wood-based materials. Accurate measurement of coating thickness is critical to ensure compliance with product warranties, optimize coating performance, and control manufacturing costs. This ASTM standard is applicable to a wide range of wood substrates, including smooth, textured, or curved surfaces and is recognized internationally for its consistency and precision.

Key Topics

• Microscopic Measurement: The standard outlines precise procedures for preparing specimens and using calibrated microscopes to measure coating thickness on wood substrates.
• Specimen Preparation: Detailed methods for properly cutting, polishing, and mounting samples are included to accommodate variations in substrate texture and coating type.
• Measurement Accuracy: Provides guidance on image analysis, calibration, and the selection of appropriate magnification for accurate dry film thickness measurement.
• Uniformity and Coating Quality: Offers methodologies to assess coating uniformity by comparing minimum and maximum DFTs, particularly on textured or uneven wood surfaces.
• Reporting Requirements: Specifies information to be documented, including average, minimum, and maximum DFT, sample descriptions, and any non-uniformities or defects observed.
• Repeatability and Reproducibility: Discusses statistical practices to ensure the reliability and consistency of measurement results across different laboratories.

Applications

• Quality Assurance in Manufacturing: Manufacturers of coated wood products use this method to verify that applied coatings meet specified thickness requirements, ensuring the desired decorative and protective properties.
• Product Warranty Verification: Specifying and confirming DFT according to product warranties helps prevent failures in service, supporting both manufacturers and end users.
• Cost Estimation: By measuring average dry film thickness, manufacturers can estimate the theoretical cost of coatings applied, optimizing material usage and reducing waste.
• Performance Evaluation: Coating companies can recommend minimum and maximum film thickness for optimal appearance and protection, improving product reliability.
• Uniformity Assessment: The method supports evaluation of coating uniformity on complex substrates, a key factor in high-performance and aesthetically-critical applications.
• Dispute Resolution: Objective measurement of DFT can resolve disagreements between buyers and sellers about whether product requirements are met.

Related Standards

• ASTM E177 - Practice for Use of the Terms Precision and Bias in ASTM Test Methods. This is referenced for defining repeatability and reproducibility criteria.
• ASTM E691 - Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method. This supports the precision and reliability claims in ASTM D5235.
• Other Coatings Standards - ASTM D5235 complements other ASTM standards related to coatings, finishes, and wood product performance.

Practical Value

ASTM D5235-18(2024) is essential for professionals involved in the production, specification, or evaluation of coated wood products. By following this standard, organizations ensure their dry film thickness measurements are accurate, reproducible, and conform to international norms. This supports quality assurance, cost control, compliance with warranties, and improved product longevity.

For manufacturers, testing laboratories, and quality control professionals seeking reliable dry film thickness measurement of wood coatings, ASTM D5235-18(2024) is the authoritative resource to establish both confidence and consistency in results.

Keywords: ASTM D5235-18(2024), dry film thickness, wood coatings, microscopic measurement, quality assurance, coating uniformity, wood products, coating standards

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D5235 − 18 (Reapproved 2024)
Standard Test Method for
Microscopic Measurement of Dry Film Thickness of
Coatings on Wood Products
This standard is issued under the fixed designation D5235; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
1.1 This test method covers the measurement of dry film
thickness of coatings applied to a smooth, textured or curved
3. Terminology
rigid substrate of wood or a wood-based product.
3.1 Definitions of Terms Specific to This Standard:
1.2 This test method covers the preparation of wood or
3.1.1 dry film thickness or DFT, n—the thickness of dried,
wood-based specimens for the purpose of microscopic mea-
cured coating film on the substrate surface which may include
surement of dry film thickness.
attached fibers but excludes free fibers that are encapsulated in
1.3 This test method suggests an analysis of dry film
the layer itself.
thickness of coatings on wood or wood-based products using a
3.1.2 edge face, n—that part of the specimen that is a plane
microscopic measurement.
perpendicular to the surface showing a cross section of the
1.4 The values stated in either SI units or inch-pound units
coating and substrate.
are to be regarded separately as standard. The values stated in
3.1.3 soak in, n—refers to a coating on a porous (wood)
each system are not necessarily exact equivalents; therefore, to
where the coating does not lie essentially on the surface of the
ensure conformance with the standard, each system shall be
wood or wood-based product, but has penetrated into the fiber
used independently of the other, and values from the two
structure of the wood or wood-based material.
systems shall not be combined.
3.1.3.1 Discussion—Wood or wood-based products are gen-
1.5 This standard does not purport to address all of the
erally of a porous nature; sometimes exhibiting uniform
safety concerns, if any, associated with its use. It is the
absorption of coatings. Frequently absorption of coatings is of
responsibility of the user of this standard to establish appro-
a non-uniform nature and influenced by localized surface
priate safety, health, and environmental practices and deter-
density differences or wood pore size. These conditions of
mine the applicability of regulatory limitations prior to use.
coating absorption are commonly referred to as soak in.
1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard-
4. Summary of Test Method
ization established in the Decision on Principles for the
4.1 A specimen of coated wood or wood-based product is
Development of International Standards, Guides and Recom-
cut to convenient size and edge face with the coating film
mendations issued by the World Trade Organization Technical
cross-section is prepared by polishing or cutting with a sharp
Barriers to Trade (TBT) Committee.
blade.
2. Referenced Documents
4.2 The prepared edge of the specimen is imaged by a
2.1 ASTM Standards:
calibrated imaging system with known magnification in order
E177 Practice for Use of the Terms Precision and Bias in
to measure the dry film thickness using an image analysis
ASTM Test Methods
computer program.
4.3 Suggestions regarding interpretation of dry film thick-
This test method is under the jurisdiction of ASTM Committee D01 on Paint
ness on porous wood or wood-based material are offered.
and Related Coatings, Materials, and Applications and is the direct responsibility of
Subcommittee D01.52 on Factory Coated Wood Products.
Current edition approved Feb. 1, 2024. Published February 2024. Originally 5. Significance and Use
approved in 1992. Last previous edition approved in 2018 as D5235 – 18. DOI:
5.1 As a base for calibration adjustment or accuracy verifi-
10.1520/D5235-18R24.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or cation of dry film coating thickness measuring instruments.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
5.2 The dry film thickness of coatings on wood or wood-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. based products is specified in written product warranties for
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5235 − 18 (2024)
proper decorative and protective performance of coatings on 8. Procedure
wood or wood-based products.
8.1 Specimen Preparation—Select the area to be measured
for coating thickness and remove it from the coated wood or
5.3 The minimum and maximum dry film thickness of
wood-based material by a suitable method. Methods of sawing,
coatings is recommended by coating companies for satisfactory
core drilling, chiseling etc., can be used. Avoid collection of
decorative and protective performance on wood or wood-based
samples near the edges of sheeting material or the ends of
products.
boards, particularly those coated in vacuum coaters, unless
5.4 The average dry film thickness of coatings on wood or
defective coating application is being investigated. For tex-
wood-based material may be used by manufacturing compa-
tured surfaces, areas representative of varying degrees of
nies to estimate the theoretical cost of applied coatings.
texture (average, extreme, minimal or non-existent) should be
5.5 The ratio of minimum to maximum dry film thickness
selected. Take care that areas of interest are not damaged.
on textured products is used as an indication of coating Cutting across wood grains is preferable in exposure of area of
uniformity.
the interest and must be conducted when a coated rough wood
surface is being tested. Inspection of the surface of interest for
5.6 Specific coated product requirements may dictate cer-
excessive damage, as well as preliminary assessment of coat-
tain film thickness determinations to be made. Agreement
ing conditions and uniformity of application should be con-
between buyer and seller may be advisable to accommodate
ducted under a microscope. Appropriate areas for coating dry
product needs relative to dry film thickness.
film thickness measurements should be selected based on
testing purpose.
6. Apparatus
8.1.1 Frequently the purpose of testing is the measurement
6.1 Microscope with Attached Digital Camera (see Note 1),
of the average coating dry film thickness for coating spread
providing sufficient magnification and image quality in the
assessment or identification of areas with excessively thick or
field of view; generally 100× to 200× magnification is suffi-
thin coating dry film thickness.
cient.
8.2 Specimen Surface Preparation—Two methods are de-
NOTE 1—Certain digital cameras can provided sufficient magnification
scribed for specimen surface preparation due to the variability
and image quality of the coating cross-section without a microscope.
in the properties of wood coatings. Method A is recommended
6.2 Source of Oblique Illumination, for the microscope.
to be used for hard coatings, for example, melamine furniture
coating or polyester coating, which are brittle at room tem-
6.3 Image Analysis Program, with capabilities for lines to
perature and could shatter in contact with a sharp blade.
be drawn along the coating/substrate interface and the coating
Method B is recommended for flexible, exterior finishes, which
surface and for calculation of the average distance between
would smear in contact with sandpaper. Such smearing may
these lines on the specimen image.
mask the appearance of voids or fracture lines in the coating
6.4 Microscope Image Calibration Tool.
and substrate and may eliminate coating/wood substrate inter-
6.5 Cutoff Saw. face details, thereby increasing measurement error.
6.6 Belt or Disc Sander. 8.3 Method A:
8.3.1 Select the desired coated area of a wood or wood-
6.7 C-Type Clamp.
based material that is to be measured for dry film thickness.
6.8 Sharp Utility Knife or Razor Blade.
With the cutoff saw, cut off a sample at least 38.1 mm [1 ⁄2 in.]
wide from this area for mounting and encapsulation in casting
7. Materials
resin (Note 2).
7.1 200 and 600-Grit Sand Paper.
NOTE 2—Some specimens could be satisfactorily prepared and tested
without mounting. Mounting may be omitted, if careful microscopic
7.2 Mould, such as a paper cup, aluminum weighing dish, or
inspection of the sanded specimen does not show any damage to the area
a 50.8 mm [2 in.] or larger diameter plastic pipe that is at least
of interest or coating edge distortion.
25.4 mm [1 in.] high.
8.3.2 Cut this specimen to a length that is at least 12.7 mm
[ ⁄2 in.] less than the inside diameter of the mold to be used.
7.3 Source of Sanding Adhesive, which is used as encapsu-
lating medium such as: 8.3.3 Place the specimen, with the sample edge to be
measured, face down and approximately centered in the mold.
7.3.1 Hot Melt Glue,
8.3.4 Prepare the sanding adhesive according to the manu-
7.3.2 Fast-Cure Acrylic Mounting Kit, or
facturer’s directions for use. A dispersed pigment may be
7.3.3 Epoxy.
added to the adhesive for better microscopic contrast between
7.4 Solvent-borne Tint Dispersion, which is compatible with
the dry film and the adhesive.
the sanding adhesive.
8.3.5 Pour the sanding adhesive around the sample in the
mold and allow to harden according to the manufacturer’s
7.5 Mineral Oil.
directions.
7.6 Automotive Red Transmission Oil.
8.3.6 Remove the mold from the hardened and encapsulated
7.7 Zinc Stearate Powder. specimen edge.
D5235 − 18 (2024)
8.3.7 Using a disc sander, belt sander or 200-grit sandpaper 8.5 Dry Film Thickness Measurement From the Image of the
mounted on a glass plate, sand the edge face of the encapsu- Edge Face of the Specimen:
lated specimen until the edge face is relatively smooth. 8.5.1 Place the polished or clean cut edge face under the
Maintain the edge face of the specimen as flat as possible microscope lens.
during sanding. Avoid heat buildup of the sanding adhesive by 8.5.2 Adjust the illuminating light at a convenient oblique
intermittent sanding if necessary. angle. Proper light angle adjustment is helpful in obtaining
8.3.8 Polish the edge face of the rough sanded specimen as good contrast along the coating surface edge, inter-coat
follows: boundaries, and coating/wood interface. This is particularly
8.3.8.1 Mount a piece of 600-grit sandpaper on a flat glass important when measuring transparent coating DFT.
plate. Rub the edge face of the rough sanded specimen over the 8.5.3 Adjust magnification. Generally 100× magnification is
600-grit sandpaper in one direction, then reverse direction by sufficient for coating DFT above 50 μm and 200× magnifica-
180° for several more rubs. Zinc stearate powder can be tion is sufficient for coating DFT above 25 μm. Lower
sprinkled on the 600-grit sandpaper or the 600-grit sandpaper magnification allows for a larger field of view which is
can be wetted with mineral oil to produce a highly polished beneficial when measuring a coating applied to rough wood
edge face free of scratches. containing peaks and valleys.
8.3.8.2 Some laboratories find that a mold for the specimen 8.5.4 Adjust contrast and lighting for best imaging of the
encapsulation with sanding adhesive is not necessary. In this coating surface edge and coating/wood interface.
case two specimens are prepared with the cutoff saw. The 8.5.5 To improve contrast between wood and coating inter-
sanding adhesive after proper mixing is generously applied to face light coat of mineral oil or automotive red transmission
the coated face of each specimen, the specimens are placed fluid may be applied.
together and a C-clamp is used to squeeze out some of the 8.5.6 Capture image of the edge face of the specimen
sanding adhesive. The C-clamp is not removed until sanding preferably with the coating film cross-section in the centre of
adhesive has hardened. the field of view.
8.5.7 Capture calibration image (Note 3) at the same mag-
8.4 Method B:
nification.
8.4.1 Select the desired area of coated wood or wood-based
material that is to be measured for dry film thickness and obtain
NOTE 3—A certified silicon test specimen, with a 10 micron grid and
1.9 micron dividing lines, for imaging system calibration could be used.
a suitable sample using the cutoff saw or other appropriate tool
Such a specimen can be supplied by Agar Scientific. Other objects with
cut off a suitable sample. There is a handling advantage if the
known dimensions measured with certified tools could also be used for
sample to be tested has a rectangular shape.
imaging and calibration.
8.4.2 To aid in the preparation of the cross-section for
8.5.8 Enter the calibration data into the image analysis
testing, the sample must be additionally cut at an acute angle
program according to the software manufacturer’s instruction.
70 to 85° with respect to the coated surface (Fig. 1). Then,
8.5.9 Determine the average coating DFT using the image
using a sharp blade in a slow continuous movement, make a
analysis program following software manufacturer’s instruc-
shallow cut parallel to the edge face with the coating cross-
tion (Note 4).
section, cutting from the coating surface into the substrate to
remove any loose wood fibers and/or damaged coating from
NOTE 4—This analysis usually involves drawing two lin
...