Powder Coating Services in China: Specifying a Finish That Actually Holds Up

Powder coating is one of those services where the sample looks fantastic and the production run curls off in the warehouse. The difference is almost never the powder brand. It is pretreatment, film thickness control, and whether anyone actually measured anything before the parts left the line.

Pretreatment is the whole ballgame

Powder adheres to a clean, chemically prepared surface. If the metal is greasy, rusty, or still carrying mill scale, the coating will sit on top of contaminant and fail exactly where the part flexes or gets chipped.

Ask what pretreatment stages the line runs. A proper multi-stage process goes through degreasing, rinsing, a conversion coating such as a phosphate or chromate-free treatment, more rinsing, and a drying stage. A shop that dips parts in one tank and calls it done will produce a coating with poor adhesion, full stop.

For aluminium, the conversion coating choice matters even more, because aluminium does not phosphating-convert the way steel does. If a shop treats aluminium exactly like steel, expect problems.

Film thickness: the number you must specify

Powder coating on a typical decorative or protective part lands somewhere in the range of 60 to 100 microns, with many interior parts aiming around 60 to 80. Too thin and you lose coverage and impact resistance. Too thick and the coating becomes brittle, chips more easily, and can crack on flexing.

Ask for film thickness measurements on the inspection report, taken at multiple points, because thickness varies across a complex part. A single reading at one spot tells you nothing about the corners and recesses.

Salt spray is a proxy, and you should know the test conditions

Salt spray hours get quoted like a universal truth, but the number depends on everything: pretreatment quality, film thickness, powder chemistry, and whether the sample was scribed or not. A scribed panel (deliberately cut to the substrate) tests a different failure mode than an unscribed one.

When a supplier claims a salt spray figure, ask for the test method, the panel preparation, the film thickness, and the hours to the first sign of blistering or creep from the scribe. Without those, the number is marketing. For demanding outdoor or coastal applications, you want a substantial figure from a genuinely tested panel, and you may want to pay for a third-party verification on a sample.

Colour, gloss, and batch-to-batch variation

Colour matching in powder is done against a standard, usually a RAL or Pantone reference number. Different powder suppliers and different bake cycles shift the result slightly. If you have matched parts coming from two different coaters, expect a visible difference unless you control the powder source and the cure schedule.

Gloss level also drifts with cure. A part baked too hot or too long will lose gloss and can yellow. Specify the gloss target and the cure window, and check a cured sample, not just a raw powder sample.

What I put in a coating specification

Substrate and its condition on arrival.
Pretreatment stages required, named.
Powder chemistry and colour reference.
Target film thickness with a tolerance range.
Gloss target and cure schedule.
Salt spray requirement with test method spelled out.
Acceptance criteria for defects: how many pinholes, orange peel, or specs per part are tolerable.

Handling after coating

A freshly coated part is soft until it fully cures and cools. Bad packing — parts stacked while warm, or resting on their coated faces — creates marring that shows up when the customer opens the box. Ask how parts are racked and packed and whether there is a cure confirmation, because a shop that rushes parts off the line to hit a shipping date will ship soft coatings.

The sample trap

Never approve a coating based only on a supplier's sample. Their sample was made carefully, possibly by a supervisor, on a good day. Order a production-representative batch, ideally a full rack, and inspect it yourself or through an inspector. The gap between sample and production is the entire reason this industry has a reputation for surprises.

Powder coating is a solved problem when pretreatment and thickness are controlled, and a recurring headache when they are not. Spend ten minutes on the spec and it stops being a lottery.