Coating Inspection Fundamentals and Surface Preparation
What the candidate must know
Inspection begins before a single abrasive bag is opened. Understand the specification hierarchy (contract, project specification, referenced standards, manufacturer's product data sheet) and the inspector's role as a verifier, not a decision-maker who overrides the spec. Know the common surface preparation standards by their intent: abrasive blast cleaning grades (visual cleanliness), surface profile/roughness (anchor pattern depth and method of measurement), and solvent/chemical cleaning for contaminants.
Key concepts: soluble salt testing (conductivity-based methods and their sampling), dust and debris assessment, ambient conditions (air and surface temperature, relative humidity, dew point), and the steel temperature margin above dew point that prevents condensation. Understand that surface profile is a range - too shallow gives poor adhesion, too deep can cause thin peaks and pinpoint rusting.
Traps in questions
- Confusing visual cleanliness grade with profile depth; they are separate acceptance criteria.
- Forgetting that the steel surface temperature, not air temperature, drives condensation risk.
- Assuming a blast grade alone removes salts; salts often require washing or specific treatment.
- Mixing up instruments: comparators for cleanliness, replica tape or profile gauges for roughness.
Rule of thumb / worked example
A common field rule is to keep surface temperature at least 3 °C (about 5 °F) above the dew point. Worked example: air temp 20 °C, RH 70 percent. Using a dew point chart gives roughly 14 °C. Add the 3 °C margin, so the steel must be at or above about 17 °C to proceed. If the steel reads 15 °C, stop and wait - condensation risk means the coating will likely fail adhesion.