The preparation sequence
- Inspect and repair: grind sharp edges and weld spatter, and address laminations. Coatings pull thin over sharp edges.
- Solvent clean (SP 1): remove oil, grease and soluble contaminants first, or blasting will drive them into the surface.
- Blast or tool clean to the specified grade, for example SP 10 near-white for immersion service.
- Check the anchor profile against the data sheet using ASTM D4417.
- Test for soluble salts where specified; chlorides cause osmotic blistering and rust under coatings.
- Remove dust with clean, dry compressed air or vacuum.
- Coat promptly, before flash rust or condensation forms, with the surface above the dew point.
Picking the cleanliness grade
The SSPC/AMPP SP standards define how clean is clean enough. As a rule, immersion and severe chemical service need SP 10 or SP 5; atmospheric service often uses SP 6 or SP 10; maintenance touch-up relies on SP 3, SP 11 or SP 15.
Anchor profile
Abrasive blasting cuts a pattern of peaks and valleys. The coating data sheet specifies a range. Too shallow reduces adhesion; too deep leaves peaks thinly covered, which can rust. Abrasive type and size control the result: angular grit cuts a sharper, deeper profile than round shot.
Environmental conditions
3 °C / 5 °FMinimum margin of steel temperature above dew point (common rule)
RHMany specs limit relative humidity during blasting and coating (often 85%)
Recoat windowCoat blasted steel the same shift, before visible rust-back
Galvanized and non-ferrous metalsSmooth galvanizing, aluminum and stainless need roughening (for example SP 16 brush-off blast) or a compatible primer. Many coatings will not bond to untreated, glossy non-ferrous surfaces.