Sa 2½ is a cleanliness grade; roughness is a separate measurement. Both have to be specified, checked and recorded — most adhesion failures start here.
Most protective coating failures that reach a dispute trace back to one layer, and that layer is usually the one nobody inspected: the prepared steel surface. Two separate properties must be specified, and they are often confused.
Cleanliness and roughness are not the same measurement
Cleanliness describes how much mill scale, rust and old coating remains. It is graded visually against reference photographs — Sa 2½ is a very thorough blast cleaning leaving only light shadows as staining. Sa 3 is blast cleaning to visually clean steel, with a uniform metallic colour.
Roughness describes the surface profile created by the abrasive. It is measured separately and quoted as fine, medium or coarse. A surface can be perfectly clean and still have the wrong profile, and vice versa.
Why both matter
- Too low a profile means the coating has too little to key into, and adhesion relies on chemical bonding alone. With thick, high-build products this is a common root cause of peeling.
- Too high a profile means the peaks poke through the primer. At nominal thickness the peaks are effectively uncoated, and early pinpoint rusting appears.
- Contamination left behind — salts, oil, dust — is the most common cause of premature blistering, and it is invisible in a visual cleanliness grade. Soluble salt testing is a separate step.
What to record
- The cleanliness grade achieved, with the standard referenced and the date.
- The roughness grade and the measurement method used.
- Ambient and steel temperature, relative humidity and dew point at the time of blasting and coating. The steel must stay at least three degrees above the dew point.
- Soluble salt content where specified, and the interval between blasting and priming.
Practical framing
The cost of surface preparation is a large share of a coating project, which is why it is the first thing squeezed. It is also the step that determines whether the other ninety per cent of the spend achieves its service life. When comparing two preparation standards, compare them against the target service life and the corrosivity category rather than against each other.