The Bresle Test Explained: Measuring Soluble Salts Before Coating

A surface can look clean and still be contaminated. Soluble salts such as chlorides, left behind from a marine atmosphere, de-icing or process exposure, draw moisture through a coating after it has cured. The result is osmotic blistering and early adhesion failure, often months after the job signed off. The Bresle test is the accepted way to measure that contamination before the first coat goes on.

What the test measures

The Bresle method measures the total density of soluble salts on a prepared surface, reported in micrograms per square centimetre or milligrams per square metre. It does not tell you which salt is present; it tells you how much soluble contamination would dissolve into water on that surface. For most coating specifications that total figure is exactly what you report against.

How the method works

The principle is simple. You isolate a known area of the surface, wet it with a measured volume of pure water, let the water dissolve any salts, then measure how conductive that water has become. More dissolved salt means higher conductivity.

1. Apply a Bresle patch to seal a known area against the surface. 2. Inject a measured volume of deionised water into the patch. 3. Work the water over the area for the time the standard specifies, so salts dissolve. 4. Withdraw the sample and read it on a conductivity meter. 5. Subtract the blank reading of your fresh water and convert to salt density.

The relevant standards are ISO 8502-6 (the sampling method) and ISO 8502-9 (the field method for conductivity), so check which your specification cites.

What you need

• Bresle patches to define and seal the test area. These are consumables, so order enough for the job.
• A conductivity meter suited to field use, ideally one that converts directly to salt density so you are not doing arithmetic on a ladder.
• Deionised or distilled water of known low conductivity, plus a clean syringe.

Some kits bundle the patches, meter and accessories together, which is the easiest way to start.

Reading the result

Compare your salt density figure against the limit in your coating specification. If it exceeds the limit, the surface needs further washing and re-testing before coating. Recording the figure, the location and the limit gives you the evidence trail an inspection report needs.

Where it fits in surface preparation

The Bresle test sits alongside dust and surface profile checks as part of confirming a surface is genuinely ready to coat. Run it after final cleaning and before priming, on the areas most likely to be contaminated.

Need a hand?

If you are setting up soluble salt testing for the first time, tell us your specification and we will recommend the patches and meter that match the standard you report against.