Short answer: the five leading causes of coating failure are inadequate pretreatment, incorrect film thickness, a faulty oven regime, part design unsuited to coating, and a powder chosen for the wrong service environment. What they share is that the fault appears not at delivery but months later.
1. Inadequate pretreatment
Symptom: corrosion creeping under the coating, blistering from the edges, film lifting easily in a tape test.
Cause: oil, salt, scale or oxide left on the surface. On such a surface the film sits mechanically but cannot form a chemical bond. Coating life is decided by the conversion layer underneath, not by the paint.
Prevention: a multi-stage pretreatment line covering degreasing, rinsing, conversion coating and drying, with regular monitoring of bath concentration and rinse-water conductivity. Blasting first for rusty or previously painted parts.
2. Incorrect film thickness
Symptom: early corrosion and edge break-up with a thin film; orange peel, colour shift and cracking on impact with a thick one.
Cause: gun settings, line speed or part geometry pushing thickness outside the target range. Film naturally thins on sharp edges.
Prevention: measuring dry film thickness at several points on the part, edges included, and agreeing the target range in writing at the start of the job.
3. A faulty oven regime
Symptom: under-cure gives a soft film that marks when wiped with solvent and adheres poorly. Over-cure gives yellowing and brittleness.
Cause: polymerisation depends on the part's own temperature being held for a set time. Even with correct air temperature, a thick-walled part reaches regime late.
Prevention: monitoring oven temperature and line speed from a single point, and lowering line speed for heavy sections.
4. Part design unsuited to coating
Symptom: uncoated areas in deep corners, liquid seeping out in the oven from closed volumes, internal corrosion in profiles without drainage.
Cause: the Faraday cage effect stops powder reaching deep corners; without drainage, pretreatment solution is trapped and escapes in the oven, ruining the film.
Prevention: drainage and hanging holes designed in, sharp edges broken, deep narrow channels avoided.
5. A powder that does not match the environment
Symptom: loss of gloss, chalking and colour fade outdoors within one to two years.
Cause: an epoxy-based powder made for indoor use, used outdoors. Epoxy resists chemicals but not UV.
Prevention: stating where the part will be used at enquiry stage; polyester or polyurethane-based powder for outdoor service.
The common denominator
Four of the five causes concern decisions made before the part enters the booth. When assessing a coating supplier, the most productive question is not "which paint do you use" but "how many stages does your pretreatment line have, and how do you monitor the oven regime".