Surface Preparation
Surface preparation is the largest single variable in coating performance. Candidates must know the SSPC/NACE abrasive-blast standards and their ISO 8501-1 equivalents: SSPC-SP 5/NACE No. 1 (white metal, Sa 3), SSPC-SP 10/NACE No. 2 (near-white, Sa 2½), SSPC-SP 6/NACE No. 3 (commercial, Sa 2), and SSPC-SP 7/NACE No. 4 (brush-off, Sa 1). Solvent cleaning (SP 1) and hand/power tool cleaning (SP 2, SP 3; St 2, St 3) are used where blast is not permitted. Near-white allows random staining on no more than 5% of the surface; commercial allows up to 33%; white metal allows none.
Two traps recur. First, visible cleanliness is not salt cleanliness. Soluble chlorides and sulfates survive blasting and cause osmotic blistering; they are measured by Bresle patch or conductivity and compared with a project limit, often 3–5 µg/cm² chloride. Second, profile is not the same as surface roughness amplitude. Blasting should produce an angular anchor pattern within the coating manufacturer's range, commonly 1.5–4 mils (40–100 µm). Too shallow gives poor adhesion; too deep can leave peaks exposed and cause pinpoint rust.
Environmental limits matter: steel temperature at least 3°C (5°F) above dew point, relative humidity within product limits, and no application to hot or contaminated steel. Edges, welds, cutouts, and back sides need stripe coats because spray deposits thin there.
Worked rule: if steel is 54°F and dew point is 50°F, the margin is 4°F, below the common 5°F minimum, so stop. If steel is 55°F, the margin is 5°F and application may proceed if the product data sheet agrees.