ISO 8502 is a family of test methods for assessing the cleanliness of a prepared steel surface. Where ISO 8501-1 deals with what you can see, ISO 8502 deals with the contamination you cannot, which is exactly why it is so important.
Dust: ISO 8502-3
After blast cleaning, fine dust settles back on the steel. Paint applied over dust bonds to the dust, not the steel. The test presses clear adhesive tape onto the surface, then compares it with reference images to give a dust quantity rating and a dust size class. The specification sets the maximum acceptable result.
Condensation risk: ISO 8502-4
Painting onto steel with an invisible film of condensation is a classic cause of poor adhesion. ISO 8502-4 gives guidance on estimating the probability of condensation from the air temperature, relative humidity and steel temperature.
The practical rule most specifications use is simple: the steel must be at least 3 °C above the dew point during preparation and application. Try our dew point calculator to check your own readings.
Soluble salts: ISO 8502-6 and ISO 8502-9
Salts such as chlorides and sulphates are invisible, and they can survive blast cleaning in the pits of corroded steel. Under a coating they draw in moisture, causing osmotic blistering and corrosion that spreads beneath the film.
The usual method is the Bresle patch: a small adhesive cell is stuck to the steel, filled with pure water, and the water is then tested for conductivity (ISO 8502-6 for extraction, ISO 8502-9 for the conductivity measurement). The result is expressed as surface density, typically in mg/m².
Acceptable limits are set by the specification. Coastal, marine and offshore work usually demands the lowest levels, and some offshore specifications are among the strictest.
Why these tests matter
Dust, salts and condensation cause some of the most expensive coating failures, because the damage often appears months later, after the scaffold has gone. A few minutes of testing at the right hold point prevents it.
How we help
We carry out dust, salt and climate testing as part of every surface preparation inspection, and investigate failures where contamination is suspected. Find out about failure investigation.
This guide summarises the standard for general understanding. Always work to the current edition and the project specification.