Surface Preparation
Surface Preparation
Surface preparation is the foundation of coating performance; inadequate preparation is a leading cause of premature coating failure. The candidate must understand abrasive blast cleaning standards (e.g., SSPC-SP 5/NACE No. 1 White Metal, SSPC-SP 10/NACE No. 2 Near-White Metal, SSPC-SP 6/NACE No. 3 Commercial Blast), power tool cleaning (SP 3, SP 11), and hand tool cleaning (SP 2). Key concepts include surface profile (anchor pattern), cleanliness (removal of mill scale, rust, oil, grease), and the importance of salt contamination testing (e.g., conductivity or chloride test).
Traps often involve the distinction between visual standards and non-visual contaminants. For example, a blasted surface may appear white metal but still have soluble salts that cause osmotic blistering. Another trap is the specification of profile: too shallow a profile may not provide adequate adhesion, while too deep a profile may cause thin spots in the coating. Questions may ask about the order of operations: pre-cleaning to remove oil and grease must be done before blast cleaning to prevent contamination of the abrasive.
A worked example: if a specification requires SSPC-SP 10 and a profile of 2-4 mils, the inspector should verify the abrasive size and blasting parameters to achieve that profile, and use a surface profile comparator or replica tape to measure. Rule of thumb: for most protective coatings, a profile of 2-4 mils (50-100 microns) is typical; always follow the coating manufacturer's data sheet.
Key points:
- SSPC-SP 5/NACE 1 is the highest level of blast cleaning (white metal).
- Soluble salts must be removed to prevent osmotic blistering.
- Pre-cleaning removes oil and grease before blasting.
- Surface profile must be within specified range; measure with comparator or replica tape.
- Follow coating manufacturer's surface preparation requirements.
- Power tool cleaning (SP 3, SP 11) is not equivalent to blast cleaning.