New powder has to go on clean metal. When a part comes in rusted, already painted or covered in mill scale, sandblasting is the surest way to get there. The association AMPP sums up its role in two points: remove what would make the coating fail, and create a surface profile that improves adhesion.
Standardized cleanliness grades
The cleanliness of a sandblasted surface is not described as "well cleaned". It is defined by standards, which the equipment maker Graco summarizes as follows: white metal (SP 5), with no traces at all; near-white metal (SP 10), where traces are tolerated on 5% of the surface; commercial blast (SP 6), up to 33%; and brush-off blast (SP 7), which removes only what no longer adheres. AMPP was born in 2021 from the merger of NACE and SSPC, hence the two acronyms still seen on specifications.
The profile: not too much, not too little
The abrasive leaves a texture of valleys and peaks on the metal. Too shallow, and the paint adheres poorly. Too pronounced, and the peaks pierce the film and rust first. For powder coating on steel, a columnist at Products Finishing recommends a profile of 1.5 to 2.5 mils, or about 40 to 65 µm, and a film thick enough to cover the peaks. The instrument maker DeFelsko adds that the number of peaks matters as much as their height.
The abrasive makes the texture
An angular abrasive, such as aluminum oxide or garnet, cuts a profile. A round abrasive, such as round shot or glass bead, cleans and leaves a satin surface without cutting in. The choice depends on the metal, the thickness of the part and the finish wanted: you do not treat thin sheet metal like a car subframe.
The race against flash rust
Freshly sandblasted steel has no protection. Depending on the humidity, a fine rust can appear within a few minutes or a few hours. The inspection firm KTA-Tator points out that a surface that has rusted again has to be cleaned a second time. So it is better to go straight from sandblasting to pretreatment to painting without waiting, which is simpler when everything is done under the same roof. Sandblasting is one of our services.
In summary
- Sandblasting cleans the metal and creates the profile the paint grips.
- Cleanliness is measured against standards: SP 5, SP 10, SP 6 and SP 7.
- For powder on steel, aim for a profile of about 40 to 65 µm, covered by the film.
- Sandblasted steel has to be treated and painted quickly, before flash rust.
