What the standard requires
SSPC-SP 7/NACE No. 4 is defined by what must be removed rather than by what must be exposed. After blasting, the surface is free of visible oil, grease, dirt, dust and loose mill scale, rust and coating. Remaining material must be tightly adherent, which the standard tests with a dull putty knife. The ISO 8501-1 grade usually compared with it is Sa 1, but the two systems are worded differently.
When it is used
- Overcoating sound old coatings. A brush-off blast removes loose and degraded paint, roughens the remaining film and prepares it for a compatible new coat.
- Galvanized and non-ferrous surfaces. Under SSPC-SP 16, a light sweep blast roughens zinc, stainless steel, aluminum or copper alloys to give a profile without removing too much metal or coating. It is a common step in a duplex system.
- Mild exposures. Where the coating data sheet and the owner's specification accept it, for example on some interior or low-corrosivity structures.
What it does not do
| Aspect | Brush-off blast | Commercial or near-white blast |
|---|---|---|
| Mill scale | Tight scale may remain | Removed |
| Rust in pits | Tight rust may remain | Removed, staining limited |
| Old coating | Tight coating may remain | Removed |
| Typical role | Overcoating, sweep blasting, mild service | New systems, severe exposures, immersion |
Because tight residue remains, brush-off blasting is not suitable for immersion linings or for coatings that require bare steel. Remaining mill scale can also crack off later and take the new coating with it.
Practical points
- Use a fine abrasive and controlled pressure on galvanized steel or thin coatings, and test a small area first.
- Confirm compatibility between the old coating and the new system before overcoating.
- Check adhesion of the remaining coating, for example with a knife test or pull-off test, as the specification requires.
- Measure the resulting surface profile when the new coating data sheet specifies one.
When an overcoat specification does not state the grade clearly, the owner, the coating manufacturer and the inspector should agree on it in writing before work starts, with a reference area kept for comparison.
Sweep and brush-off blasting are part of our abrasive blasting service.