What the standard requires
SSPC-SP 6/NACE No. 3 requires the removal of all visible residue: mill scale, rust, coating, oxides and foreign matter. What remains can only be staining, meaning light shadows, slight streaks or discolorations caused by rust, mill scale or old coating. The staining limit is judged on each unit area of 9 in² (a 3 in by 3 in square), not as an average over the whole surface. Any residue, as opposed to a stain, fails the grade.
Where it sits among blast grades
| Grade | SSPC/NACE | ISO 8501-1 (approximate) | Staining allowance |
|---|---|---|---|
| White metal | SSPC-SP 5/NACE No. 1 | Sa 3 | None |
| Near-white metal | SSPC-SP 10/NACE No. 2 | Sa 2½ | 5 % of each unit area |
| Commercial | SSPC-SP 6/NACE No. 3 | Sa 2 | 33 % of each unit area |
| Industrial | SSPC-SP 14/NACE No. 8 | No direct equivalent | Some tightly adherent residue allowed |
| Brush-off | SSPC-SP 7/NACE No. 4 | Sa 1 | Tightly adherent material may remain |
ISO 8501-1 and SSPC are written differently, so the comparison is approximate. The specification should name the standard that governs.
When it is specified
- structural steel and equipment in atmospheric exposure, when the data sheet accepts SSPC-SP 6;
- maintenance painting where the coating system is designed for this level;
- primers that tolerate some staining, as stated by the manufacturer.
Immersion linings, zinc-rich primers in severe exposures and thermal spray usually require a higher grade. The coating data sheet takes precedence: it states the minimum grade for each product and service.
Inspection points
Commercial blast is checked visually against the written definition, with SSPC-VIS 1 photographs as a guide. The same inspection normally covers surface profile per ASTM D4417, dust, soluble salts where specified, and conditions relative to the dew point. Rerusting before coating can bring the surface below the grade, so the time between blasting and priming is kept short.
On heavily pitted steel, extra blasting time is usually needed in the pits, since material left at the bottom of a pit is a residue, not a stain, and fails the grade.
Blast grades are selected and verified in our abrasive blasting service.