Sa grades describe how clean the surface is. Rz describes how rough it is. A blast job is accepted against both, and Rz is the one that causes arguments, because two abrasives can both reach Sa 2½ while producing profiles 40 μm apart.
| Grade | Meaning | Typical Rz | Usually specified for |
|---|---|---|---|
| Sa 1 | Light blast | 15–35 μm | Secondary structures, surfaces that will not be coated |
| Sa 2 | Thorough blast | 30–55 μm | General industrial coating |
| Sa 2½ | Very thorough, near-white | 40–75 μm | Heavy-duty systems, offshore, tank interiors |
| Sa 3 | White metal | 60–100 μm | High-performance coatings, stainless before passivation |
Ranges are typical, not fixed by the standard. ISO 8501-1 defines the cleanliness grades; profile is a separate measurement, usually by ISO 8503 comparator or stylus.
Coarse and fine do different jobs
| Coarse, 30/36# | Fine, 120# and up | |
|---|---|---|
| Thick coating and mill scale | Removed quickly | Slow, sometimes not at all |
| Profile produced | 60–100 μm | 25–40 μm |
| Cleaning pits and valleys | Poor — leaves embedment | Better |
| Surface embedment | High: fractured abrasive stays in the profile | Low |
| Passes over the same area | Fewer | More |
The common failure is a single coarse grade run at high pressure. The profile is deep, the coating goes on, and six months later the owner finds rust in a pattern that follows the nozzle. The abrasive fractured in the nozzle and left fragments inside the profile that the coating could not wet.
The other failure is the opposite: a fine grade chosen for a good-looking surface, then a coating specified for a 75 μm profile goes on top of a 30 μm one. Adhesion test passes in the shop and fails after a season outdoors.
What actually moves the profile
- Pressure — the largest single lever, and the one most often left at the factory setting.
- Angle — below about 45° you are cutting rather than peening, and embedment rises.
- Stand-off — too close gives an uneven pattern, too far wastes abrasive.
- Grain size and shape — angular crushed garnet profiles deeper than rounded media at the same size.
- Nozzle wear — a worn nozzle widens the pattern and drops velocity without changing the gauge reading.
If the profile shifts after a supplier change, read the PSD before touching the pressure gauge. An “80 mesh” with 15% oversize profiles deeper than one without, at identical settings and identical consumption.
Salt and dust are separate tests
ISO 8502-3 covers residual dust before coating. ASTM D4940 and ISO 11127-6 cover conductivity of water-soluble contamination. NORSOK M-501 puts the offshore threshold at 20 mg/m² of soluble salts. Recycled abrasive accumulates salts over cycles, which is why reuse rate and salt content have to be read together — a high reuse figure on a system with weak dedusting is a coating-adhesion problem waiting to happen.
What to write into the acceptance clause
- Sa grade by ISO 8501-1, assessed on the workpiece, not on a test coupon.
- Profile Rz with the measurement method and the comparator used.
- Residual dust by ISO 8502-3, and the chloride or conductivity limit if the part is coated.
- Abrasive free silica, total chlorides and conductivity per batch, printed on the COA.
- Reuse policy: how many cycles, and who tests the recycled stock.