Surface Preparation: Cleanliness, Profile, and Standards
Core concepts
Coating performance depends on a properly prepared substrate. Candidates must know the sequence: remove visible contaminants (oil, grease, salts, dust), then achieve the specified cleanliness and profile. Solvent cleaning removes grease but not salts or mill scale; detergent washing addresses salts; abrasive blast cleaning removes scale, rust, and old coating while creating anchor profile.
Standards to recognize
Know the intent of SSPC/NACE abrasive blast cleanliness grades (White Metal, Near-White, Commercial, Brush-Off) and the ISO Sa grades that parallel them. Understand that surface profile (anchor pattern) is measured separately from cleanliness; a surface can be clean but have insufficient profile. Profile is typically assessed by replica tape or comparator, and its depth should fall within the coating manufacturer's range.
Traps in questions
Common distractors: assuming a higher cleanliness grade automatically provides profile; confusing profile with roughness caused by corrosion pitting; or treating solvent cleaning as sufficient before blasting. Another trap is salt contamination: invisible chloride residue causes premature failure, so conductivity or test-kit checks may be required. Also watch for questions that swap the order of preparation steps.
Worked rule of thumb
Match the blast grade to the service environment: Near-White for severe immersion or aggressive service, Commercial for moderate atmospheric exposure, per the coating data sheet. Always verify profile with replica tape and record readings before applying primer.