1. How Surface Preparation Grades Are Defined
Surface preparation grades follow GB/T 8923.1 (corresponding to ISO 8501-1). The common grades are Sa1, Sa2, Sa2.5 and Sa3; the higher the number, the more thorough the cleaning.
2. What Each Grade Means
- Sa1: light blast cleaning, removing loose mill scale and rust.
- Sa2: thorough cleaning; the surface is free of visible oil, mill scale and rust, with only slight firmly adherent residues allowed.
- Sa2.5: very thorough cleaning; practically no oil, mill scale, rust or foreign matter remains, with only very slight traces allowed. This is the most common grade for coating systems.
- Sa3: blast cleaning to a bare metal finish, used where coating adhesion requirements are very high.
3. Shot Blasting vs Sand Blasting
Shot blasting propels steel shot at high speed and suits batch processing of members; sand blasting (grit blasting) is chosen according to media and equipment conditions. The process is determined by member shape, batch size and the coating specification.
4. Matching the Grade to the Coating System
The grade must match the primer, intermediate and top coat system: commonly Sa2.5 for regular plant members; marine, bridge and highly corrosive environments may require a higher grade and a heavier protective system. Surface profile should also match the paint type.
5. How to Specify Requirements When Quoting
State the required preparation grade, surface profile range, coating system (paint schedule and dry film thickness), and any required overcoating interval after blasting in the drawings or technical requirements; special requirements can be discussed.
If you have members to treat or want to review our fabrication capabilities, see our steel fabrication service page, or send your drawings for a review and quote.