Surface Preparation Standards and Methods
What the exam expects
Surface preparation is where most coating failures originate, so questions are dense here. Know the visual standards by name and what each represents: SSPC-SP 1 (solvent cleaning), SP 2 (hand tool), SP 3 (power tool), SP 5/NACE No. 1 (white metal), SP 6/NACE No. 3 (commercial blast), SP 10/NACE No. 2 (near-white), and SP 11 (power-tool bare metal). Pair these with ISO 8501-1 grades (Sa 1, Sa 2, Sa 2½, Sa 3; St 2, St 3) and be ready to map near-white blast to Sa 2½.
Understand abrasive properties: angular vs. round, hardness, size, and cleanliness (SSPC-AB 1 for recycled ferrous abrasive). Know surface profile targets and measurement (comparator discs, replica tape for field work) and that profile must suit film thickness. Recognize soluble salt testing (Bresle patch conductivity, chloride-specific kits) and typical acceptance thresholds.
Traps
- Equating SP 6 with SP 10; SP 10 permits only very light staining, SP 6 permits more.
- Thinking profile is a cleanliness grade - it is roughness, not removal of contaminants.
- Forgetting that ambient conditions (dew point, steel temperature, humidity) are part of preparation, not just application.
- Assuming recycled abrasive is acceptable without checking for oil or salt contamination.
Rule of thumb
Steel surface temperature should be at least 3 °C (5 °F) above dew point before blasting or coating; below that, condensation films form and adhesion suffers.
Worked example
A specification calls for near-white blast to ISO Sa 2½ with 50–75 µm profile. The correct pairing is SSPC-SP 10/NACE No. 2, verified by comparator or replica tape, and abrasive size selected to achieve that profile rather than maximum roughness.