Surface Preparation
Cleanliness and profile are two different requirements
Surface preparation produces both a cleanliness level (what is removed) and a profile or anchor pattern (what is created). Questions frequently blur the two. A surface can be white-metal clean and still fail because the profile is too low, or carry a perfect profile and still fail because soluble salts remain. Blasting does not remove oil - solvent cleaning (SSPC-SP 1) must precede it, or the abrasive drives contamination into the substrate.
The standard ladder
Memorize the pairs:
- SSPC-SP 5 / NACE No. 1 = white metal; ISO 8501-1 Sa 3
- SSPC-SP 10 / NACE No. 2 = near-white; Sa 2½ (most commonly specified)
- SSPC-SP 6 / NACE No. 3 = commercial blast; Sa 2
- SSPC-SP 7 / NACE No. 4 = brush-off blast
- SSPC-SP 2 / SSPC-SP 3 = hand and power tool; St 2, St 3
- SSPC-SP 11 = power-tool cleaning to bare metal
Trap: "Sa 2½" is near-white, not white metal; "SP 6" is commercial, not near-white. Distractors swap adjacent grades.
Verifying the work
Check abrasive type and size against the profile specified, nozzle pressure typically 90–100 psi at the nozzle with the machine running, a 75–90° impingement angle, and roughly 18–24 in standoff. Verify profile with replica tape, a comparator, or a stylus instrument; verify residue with the Bresle method for soluble salts, the tape method for dust, and visual standards for rust grades. Blast only when steel temperature is at least 3°C (5°F) above dew point.
Worked example: a specification calls for SSPC-SP 10/NACE 2 with a 2.0–3.5 mil profile. The contractor produces a uniformly gray surface with faint mottling. That appearance is acceptable for near-white; the real questions are whether shadowing or residue remains and whether the profile tape reads within range.