Define the sampled surface

Identify whether the test concerns exposed metal, remaining coating or another material, because the method and interpretation must fit the actual boundary. DeFelsko describes extraction-based assessment of soluble salts on steel surfaces. A result from one accessible flat region should not automatically represent a nearby crevice or irregular repair edge. Sampling can be useful while still leaving those regions unresolved.

Keep units and method together

A surface-density result and a solution-concentration result are different quantities. Interpretation depends on the extraction area, volume and method, including relevant blank or background effects. Do not compare numbers from unlike methods without resolving those definitions. A conductivity-based test does not uniquely identify each dissolved chemical, and it cannot prove the absence of oil, insoluble dust or other contaminants that require different assessment.

If a report gives milligrams per square metre and another gives a concentration in the extract, compare their definitions before ranking cleanliness. A numerical difference can reflect the reporting boundary rather than a real difference in the amount of contamination on the surface.

Apply the actual repair criterion

Ask the coating supplier or responsible inspector which requirement governs the identified substrate and repair system. No universal allowable salt concentration can be transferred from an unrelated marine or structural specification. A compliant sample does not establish adhesion, cure or substrate strength. No MARSTON coating repair criterion is asserted here; the result should support a bounded cleanliness decision as one part of the full repair assessment.