When you ask for a blasted surface, “clean” is not a specification. The preparation grade is.
The standard
Assessment follows ISO 8501-1, also known as the Swedish standard. The method is visual: you compare the actual surface against reference photographs.
It is not an instrumental measurement. It is a comparison, made by a person, in good light.
The four grades
SA 1 — light blast cleaning. The loosely adhering layer is removed. Firmly adhering mill scale and rust may remain on the surface. Adhering means they cannot be lifted with a blunt putty knife.
SA 2 — thorough blast cleaning. Almost all visible contamination is removed.
SA 2.5 — very thorough blast cleaning. The surface is clean to roughly 96%. Only light traces remain, as stains or streaks.
SA 3 — blast cleaning to visually clean metal. Complete cleaning, around 99%. The surface has a uniform metallic appearance.
What you specify, in practice
The grade is chosen according to what comes next, not according to how nice it looks.
SA 2 is enough for many ordinary industrial painting applications.
SA 2.5 is the one most often specified for protective coatings.
SA 3 is specified for special coatings, aggressive environments, and parts that will no longer be accessible for maintenance.
Each step up costs more time, more abrasive and more energy. Do not specify SA 3 if the job needs SA 2.
The second criterion
Cleanliness is not the only thing assessed. The second is surface roughness — the profile left by the abrasive.
It decides how well the coating anchors. A surface that is clean but too smooth can hold paint worse than one with the right profile.
Cleanliness and roughness are specified together. One without the other does not describe the result.
Related reading
- How to choose shot size and hardness
- Sandblasting
- Industrial painting systems: automatic, manual and booths
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