The Powder Coat That Chipped: Surface Prep, Not the Paint
A welded frame powder-coated. The paint chipped off at the edges within a month. The customer thought the powder coat was bad. The powder coat was fine. The surface prep (shot blasting) was inadequate. This is about powder coating surface preparation, not about the paint.
Why powder coat fails
Powder coating bonds to a clean, properly prepared surface. If the surface has oil, rust, or mill scale, the powder doesn’t adhere. It chips off at the first impact. The quality is in the prep, not the powder.
The prep steps
- Clean: Degrease with a solvent or alkaline wash. Remove all oil, coolant, and fingerprints.
- Strip: Shot blast or sand blast to remove mill scale, rust, and weld scale. The surface should have a uniform matte finish.
- Phosphate (optional): A zinc phosphate conversion coating improves adhesion for outdoor use.
- Dry: Oven dry at 120°C. No moisture before powder application.
- Powder: Electrostatic application. The powder sticks to the grounded part.
- Cure: Oven at 180-200°C for 20 minutes. The powder flows and cross-links.
What we found
The frame was degreased but not shot-blasted. The mill scale (dark, flaky surface on hot-rolled steel) was still on the surface. The powder coat adhered to the scale, not the steel. When the frame was bumped, the scale came off, taking the paint with it. We re-blasted the frame, stripped the old powder, and re-coated. The new coat didn’t chip.
The edge prep
Sharp edges are the first place powder chips. The powder film is thin at edges (the electrostatic field pulls away). We broke all sharp edges with a file or grinder. A 0.5 mm radius at every edge. The powder builds up on the radius instead of the sharp corner. No more edge chipping.
The powder coat rule: If powder coat chips, check the prep. Degrease, shot blast, phosphate, dry, then coat. Break sharp edges. The paint that chipped wasn’t bad — the surface wasn’t prepared for it. Powder coating lives or dies on the prep, not the powder.
