ISO 8502-16:2026

• Abstract:

  This document specifies a method of extracting and analysing water-soluble contaminants from a surface by use of filter paper which can be attached to any surface, regardless of its shape (flat or curved) and its orientation (facing in any direction, including downwards). The method described in this document is suitable for use in the field to determine the presence of water-soluble contaminants before painting or a similar treatment. NOTE The extraction method can give a false conductivity result or not take all the water-soluble material off the surface because of: (1) soluble materials hiding in the crevices or under folds of metal; (2) soluble materials under corrosion layers, passivation layers produced by corrosion inhibitors, oil, grease, or other non-visible thin films.

• Status: Published
• Publication Date: 03-Nov-2026
• ICS: 25.220.10 - Surface preparation
• Technical Committee: ISO/TC 35/SC 12 - Preparation of steel substrates before application of paints and related products
• Drafting Committee: ISO/TC 35/SC 12 - Preparation of steel substrates before application of paints and related products
• Current Stage: 6060 - International Standard published
• Start Date: 04-Nov-2026
• Completion Date: 04-Nov-2026

ISO 8502-16 is an ISO standard for assessing the cleanliness of steel substrates before painting and related products. It specifies a field method that uses saturated filter paper to extract water-soluble contaminants from steel surfaces for conductivity analysis. Engineers, inspectors, quality managers and buyers use it when they need a practical check for soluble salts before paint or a similar treatment is applied.

What does ISO 8502-16 specify?

ISO 8502-16 specifies a method for extracting and analysing water-soluble contaminants from a surface by means of saturated filter paper. The method is intended for field use and works on flat or curved surfaces in any orientation, including downward-facing surfaces.

The document is organized into these clauses and annexes:

Annex	What it covers
Annex A	Temperature compensation
Annex B	Test procedure

The main clauses cover the scope, normative references, principle, apparatus and materials, procedure, analyses, calculation of salt density, and test report. In practice, that means the standard tells users what to use, how to sample, how to measure conductivity, and what to report.

What are the key requirements of ISO 8502-16?

ISO 8502-16 focuses on a simple extraction-and-measurement workflow: place saturated filter paper on the test area, transfer soluble contaminants into the paper, and analyse the resulting solution by conductivity. The details matter because the result is used to judge whether the steel surface is clean enough before coating.

Sampling and contamination control

Clause 6.1 says the test area should be representative of the whole surface. Clause 6.2 also requires precautions against cross contamination and thorough cleaning of equipment before use or re-use. In practice, this keeps the result tied to the surface being checked, not to dirt carried in by tools or hands.

Materials and water

Clause 5 lists the required items: filter paper, a reusable syringe, fresh water, disposable vinyl gloves, tweezers and a conductivity meter. Fresh water complying with ISO 3696 grade 3 should be used, and its conductivity should be checked first. That matters because the test is sensitive to contamination from the water itself.

Extraction method

Clause 6.2 describes how the filter paper is wetted, pressed firmly onto the surface, and left in contact for the time required by the meter manufacturer. The paper must follow the surface contours and all transferred water must be retained. This is what makes the method usable on irregular steel surfaces and keeps the extracted sample valid.

Blank test and analysis

Clause 6.3 requires a blank test before each series of tests and whenever a new batch of paper or water is used. Clause 7 then calls for conductivity measurement with a suitable conductivity meter designed for saturated filter paper. In practice, the blank value is subtracted from the surface reading, so the reported result reflects the surface and not the water or paper.

Salt density and reporting

Clause 8 converts conductivity data into total surface density of soluble salts using a formula based on conductivity difference, water volume and test area. Clause 9 requires a report that identifies the standard, meter type, water type, blank values, contact time, temperature or temperature compensation, conductivity results normalized to 25 °C, paper batch number, date and unusual features. This gives users the traceability needed for quality records and comparisons between sites.

What terms does ISO 8502-16 use?

ISO 8502-16 uses a small set of practical terms that are central to the method:

• Water-soluble contaminants - soluble materials removed from the steel surface by the wet filter paper and then measured in solution.
• Saturated filter paper - the wetted paper used to pick up contaminants from the test surface.
• Blank test - a check on the paper and water before surface testing, used to correct subsequent readings.
• Conductivity meter - the instrument used to measure the conductivity of the solution held in the filter paper.
• Salt density - the calculated surface amount of soluble salts reported from the conductivity measurement.
• Temperature compensation - the correction used to express conductivity as an equivalent value at 25 °C.

Who uses ISO 8502-16?

ISO 8502-16 is used by people who prepare, inspect or accept steel surfaces before protective coating. That includes field inspectors, coating contractors, quality teams, laboratory and site technicians, and buyers or specifiers who need a consistent check for soluble salt contamination.

It is relevant in paint and related product applications where soluble contaminants can affect coating performance. The method is especially useful when a quick field test is needed on surfaces that are not easy to sample with a rigid cell.

Which standards are used with ISO 8502-16?

ISO 8502-16 cites ISO 3696 for the water used in the test. ISO 3696 contributes the water quality basis for the solvent, which is important because the test measures very small conductivity changes.

The introduction also places ISO 8502-16 within the broader preparation and assessment series:

• ISO 8501 - visual assessment of surface cleanliness
• ISO 8502 - tests for the assessment of surface cleanliness
• ISO 8503 - surface roughness characteristics of blast-cleaned steel substrates
• ISO 8504 - surface preparation methods

Together, these standards support different parts of steel surface preparation, from visual assessment and roughness to cleanliness testing and preparation methods.

What does the ISO 8502-16 document contain?

ISO 8502-16 contains the test principle, equipment list, step-by-step field procedure, conductivity analysis, calculation of salt density and reporting requirements. It also includes two informative annexes: one on temperature compensation and one with a visual test procedure.

Annex A explains how to report conductivity at 25 °C when the meter does not compensate automatically. Annex B illustrates the practical sequence: saturate the paper, place it on the surface, then transfer the sample to the measuring device with tweezers.

The document also gives guidance on representative test area selection, the need to prevent water loss during handling, and the use of a blank test to correct readings. For users, that means the standard is not just a principle statement - it is a complete field method for sampling and reporting soluble contamination on steel.

International
Standard
ISO 8502-16
First edition
Preparation of steel substrates
2026-11
before application of paints and
related products — Tests for the
assessment of surface cleanliness —
Part 16:
Extraction of water soluble
contaminants for analysis, the
saturated filter paper method
Préparation des subjectiles d'acier avant application de peintures
et de produits assimilés — Essais pour apprécier la propreté
d'une surface —
Partie 16: Extraction des contaminants solubles dans l’eau pour
analyse, méthode du papier filtre saturé
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus and materials . 2
5.1 Filter paper .2
5.2 Reusable syringe .2
5.3 Fresh water .2
5.4 Disposable vinyl gloves .2
5.5 Tweezers .2
5.6 Conductivity meter .2
6 Procedure . 2
6.1 Selection of test surface .2
6.2 Extraction by filter paper .2
6.3 Blank test .3
7 Analyses . 3
8 Calculation of salt density . 4
9 Test report . 4
Annex A (informative) Temperature compensation . 5
Annex B (informative) Test procedure . 6
Bibliography . 8

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 35, Paint and varnishes, Subcommittee SC 12,
Preparation of steel substrates before application of paints and related products.
[1]
A list of all parts in ISO 8502 (all parts) 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 atwww.iso.org/members.html.

iv
Introduction
The performance of protective coatings of paint and related products applied to steel is significantly
affected by the state of the steel surface immediately prior to painting. The principal factors that are known
to influence this performance are:
a) the presence of rust and mill scale;
b) the presence of surface contaminants, including salts, dust, oils and greases;
c) the surface profile.
[2] [1]
The series of International Standards ISO 8501 (all parts) , ISO 8502 (all parts) and ISO 8503 (all parts)
[3] [4]
have been prepared to provide methods of assessing these factors, while ISO 8504 (all parts) provides
guidance on the preparation methods that are available for cleaning steel substrates, indicating the
capabilities of each method in attaining specified levels of cleanliness. These International Standards do not
contain recommendations for the protective coating systems to be applied to the steel surface. Nor do they
contain recommendations for the surface quality requirements for specific situations, even though surface
quality can have a direct influence on the choice of protective coating to be applied and on its performance.
Such recommendations are found in other documents such as national standards and codes of practice.
Users of these International Standards are expected to ensure that the qualities specified are:
— compatible and appropriate both for the environmental conditions to which the steel will be exposed
and for the protective coating system to be used;
— within the capability of the cleaning procedure specified.
These series of International Standards referred to above deal with the following aspects of preparation of
steel substrates:
[2]
ISO 8501 (all parts) — Visual assessment of surface cleanliness;
[1]
ISO 8502 (all parts) — Tests for the assessment of surface cleanliness;
[3]
ISO 8503 (all parts) — Surface roughness characteristics of blast-cleaned steel substrates;
[4]
ISO 8504 (all parts) — Surface preparation methods.
[1]
This is one of many parts of ISO 8502 (all parts) that specify tests for the assessment of surface cleanliness.
Relating to such tests, there are several methods for extracting soluble contaminants on surfaces to be
painted, which are then used for subsequent analysis. One of these methods is based on the swabbing of
comparatively large test surfaces. This technique provides average values of the contamination present, but
it can conceal localized concentrations of contaminants. Also, it is possible that swabbing does not ensure
sufficient penetration to dissolve all the deep-seated contamination such as ferrous salts.
There are other methods which use small cells for the liquid used to remove and collect the surface
contaminants. The cells are attached to test surfaces where soluble contaminants can be expected. This
closed cell technique usually provides more accurate, point values of the contamination present than the
swabbing method.
This document describes a further, inexpensive field test using saturated filter paper to extract the soluble
salts from the surface. This method combines the benefits of testing a large area with the benefit of greater
accuracy and point measurements
...