What the standard requires
SSPC-SP 10/NACE No. 2, now maintained under AMPP, sets a visual cleanliness criterion. After blasting, the surface must show no visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products or other foreign matter. The only tolerance is staining: light shadows, slight streaks or minor discolorations caused by stains of rust, mill scale or previously applied coating.
Two details make the grade stricter than it sounds:
- The unit area is the unit of judgement. Staining is limited to 5 percent of each unit area of 9 in² (a 3 in by 3 in square), not averaged over the whole surface.
- A stain is not a residue. Tight mill scale, rust at the bottom of a pit or a spot of old primer is a residue, and any residue fails the grade.
Where it sits among blast grades
| Standard | Grade | Staining allowed | Approximate ISO 8501-1 equivalent |
|---|---|---|---|
| SSPC-SP 5/NACE No. 1 | White metal | None | Sa 3 |
| SSPC-SP 10/NACE No. 2 | Near-white metal | 5 % of each unit area | Sa 2½ |
| SSPC-SP 6/NACE No. 3 | Commercial | 33 % of each unit area | Sa 2 |
| SSPC-SP 7/NACE No. 4 | Brush-off | Tightly adherent material may remain | Sa 1 |
The ISO equivalences are approximate. ISO 8501-1 judges cleanliness against reference photographs for each initial rust grade, while SSPC relies on written definitions and a staining limit; the two can disagree on the same surface. A specification should name the governing system, as discussed in ISO 8501 vs SSPC: which governs. SSPC-VIS 1 provides reference photographs to support visual assessment.
SP 10 is the usual minimum for immersion service, tank and vessel linings and many high-performance systems in severe atmospheric categories, where any residue left under the coating becomes a blister or corrosion initiation site.
What the grade does not cover
SP 10 defines visible cleanliness only. It does not set the anchor profile, which is specified and measured separately as surface profile; it does not limit invisible contaminants such as soluble salts, which are tested to ISO 8502-6 and ISO 8502-9; and it does not cover dust, assessed to ISO 8502-3. Nor does it survive time: blasted steel that rusts back before coating no longer meets the grade and must be re-blasted to it. A complete preparation clause therefore names the cleanliness grade, the profile range, the salt and dust limits, and the maximum delay before priming.