Powder Coating Troubleshooting: 8 Common Defects

This is the fast version: eight defects that account for most powder coating rejections on an Indian shop floor, what each one looks like, and the single most likely cause. If you need the full diagnostic logic: how to tell two similar-looking defects apart, and how to prove a root cause before you change a process parameter, read Powder Coating Defects: The Complete Diagnostic Guide.

1. Fisheyes

What you see: small circular craters, usually 1–5 mm across, with a raised rim and bare or thin substrate at the centre. They appear during flow-out, not after.

Most likely cause: silicone or oil contamination on the substrate or in the compressed air line. Silicone is the usual culprit because it lowers surface tension locally and the molten powder pulls away from that spot.

First check: the air line moisture trap and the oil separator on the compressor. Then check whether any silicone-containing product, mould release, WD-40, hand cream, aerosol lubricant, is used anywhere upstream.

2. Orange peel

What you see: a dimpled surface texture resembling citrus skin. Gloss is usually still present but the reflection is distorted.

Most likely cause: film thickness outside the powder's designed window, or a cure ramp that is too fast for the powder to level before it gels.

First check: measure dry film thickness at several points. Most decorative thermosets level properly between 60 and 80 microns. Below that the powder does not have enough mass to flow out; above it, the surface skins before the bulk melts.

3. Faraday cage effect

What you see: thin or bare coverage inside corners, channels, recesses, tube ends and behind flanges, while flat faces are coated normally.

Most likely cause: electrostatic field lines terminate on the nearest projecting edge instead of reaching into the recess, so powder never arrives at the internal corner.

First check: reduce the applied kV, 40–50 kV rather than 90 kV, and increase the powder-to-air ratio so more powder arrives mechanically rather than electrostatically. Coat recesses first, flats second.

4. Outgassing pinholes

What you see: a scatter of very fine pinholes or small blisters, most often on castings, galvanised steel or weld seams rather than on plain sheet.

Most likely cause: gas escaping from porosity in the substrate during cure, punching through the coating while it is still molten.

First check: pre-bake the parts above cure temperature before coating, so the gas leaves before the film exists. For castings and hot-dip galvanised work, a degassing powder formulated with a benzoin additive is the correct specification.

5. Poor adhesion and peeling

What you see: the coating lifts in sheets or flakes, often from an edge or an impact point. A cross-hatch test scores worse than 4B.

Most likely cause: pre-treatment failure. Either the surface was not clean, or the conversion coating was not formed, or the part was contaminated between rinse and oven.

First check: the pre-treatment bath concentrations and the final rinse conductivity. Adhesion failure is almost never a powder problem. See Pre-Treatment Before Powder Coating.

6. Blistering

What you see: raised bubbles under the film, appearing hours or days after coating rather than immediately.

Most likely cause: trapped moisture or residual salts under the coating drawing water through the film osmotically.

First check: whether parts are fully dry before entering the coating booth, and whether the final rinse is de-ionised. Residual chloride or sulphate from an incomplete rinse is a common cause on humid days.

7. Colour and gloss variation between batches

What you see: two runs of the same shade that do not match under the same light, or a gloss reading that drifts between batches.

Most likely cause: cure variation. Under-cure and over-cure both shift gloss, and over-cure yellows lighter shades.

First check: metal temperature, not oven set-point. A thermal profile logged on the part is the only reliable measurement. Most thermosets are specified at 180–200 °C metal temperature held for 10–15 minutes.

8. Chalking and fade on outdoor parts

What you see: a powdery white bloom on the surface and a visible loss of colour after months to a couple of years outdoors.

Most likely cause: the wrong resin chemistry. Epoxy and epoxy-polyester hybrids are not UV stable; the binder degrades at the surface and releases pigment.

First check: the specification, not the process. Anything permanently exposed to sunlight needs a pure polyester or polyurethane. See Epoxy Polyester vs Pure Polyester.

Where to go next

Two defects in this list can look almost identical in a photograph, fisheyes and outgassing pinholes in particular, and treating one as the other wastes a shift. The complete diagnostic guide works through the distinguishing tests for each pair, plus the sequence to follow when more than one defect appears on the same part.

If you are seeing a defect you cannot place, Oracle Industries can review photographs and process parameters with you. Get in touch.