Short answer: dry film thickness in powder coating generally runs in the 60–120 micron range. A thin film does not form an adequate barrier against corrosion; an excessive film becomes brittle, shows orange peel and shifts colour. Thickness is verified by measurement, not by looking.
What exactly is measured?
The measured value is dry film thickness: the thickness of the coating from the metal surface, after the part leaves the oven and the film has polymerised. The unit is the micron (μm; 1000 microns = 1 mm). Magnetic and eddy-current instruments read this value without damaging the part.
Why a thin film is a problem
The corrosion function of a coating is to form a barrier. As the film thins, the barrier becomes porous and moisture and salt reach the metal. The symptom usually appears not at delivery but at the end of the first winter, as rust starting from the edges.
Film thins most at sharp edges and corners. So if measurement is taken only on flat faces, the genuinely weak point of the part has not been checked at all.
Why a thick film is a problem
- Brittleness. A thick film cracks under impact and bending, and a crack is a direct route in for corrosion.
- Appearance. As thickness rises the surface turns to orange peel and gloss comes out different from expectation.
- Colour. In low-opacity colours especially, tone shifts with thickness; the same RAL code looks different on two parts.
- Tolerance. On mating surfaces and threads, a 150-micron film can prevent assembly.
- Cost. Excess thickness is excess powder consumption, one for one.
How much should it be?
The target range follows the job: for a part used indoors the lower bound is enough, while a part going outdoors or into a corrosive environment moves the range up. Where a customer specification exists, it governs. Where none exists, the range should be discussed at the start of the job and put in writing — "coat it well" is not a measurable acceptance criterion.
What to state in your enquiry
- The target film thickness range, or the specification to be met.
- Which surfaces measurement is taken from (flat faces only, or edges included).
- Whether a measurement record is required with delivery.
- Mating faces, threads and areas requiring tolerance.
Settling these four items up front removes almost every thickness dispute that would otherwise surface after delivery.