Which powder coating defects should a receiving inspector reject, and what causes each?

What are the common powder coating defects, and how do you recognize each one?

The defects that come up most at receiving fall into eight groups: pinholes, blisters or bubbles, fish eyes and craters, chipping or flaking, thin or bare edges, orange peel, embedded dirt, and over-bake discoloration. Each has a recognizable look and location pattern, and the pattern is the best first clue to the cause.

A pinhole is a tiny hole, often with a raised rim, that may or may not reach the metal. A blister is a dome of film lifted off the substrate or off a lower coat; if it is already broken it can look like a pinhole with a ring around it. A fish eye or crater is a round depression where the melted powder pulled away from a spot of contamination, usually wider and shallower than a pinhole and sometimes with a visible dot in the center. Chipping is film that breaks away under impact or at an edge, and if large flakes lift cleanly from bare metal, the real problem is adhesion rather than handling.

Edge defects deserve their own line on the inspection sheet. Melted powder is pulled away from sharp edges by surface tension, so a knife edge left by machining often ends up with thin or no film, and inside corners and deep recesses tend to be thin for a different reason: the electrostatic field concentrates on outer edges and does not reach into the recess, which coaters call the Faraday cage effect. Orange peel is a dimpled texture like the skin of an orange; it is normal to some degree in powder, so it is a defect only when it exceeds an agreed sample. Dirt or “seeds” are raised specks of foreign matter or gelled powder trapped in the film, and over-bake shows as yellowing, loss of gloss or a color shift, most visible on white and light colors.

How do you diagnose a powder coating defect at the receiving bench?

Start with the location pattern, then measure: a defect that repeats in the same place on every part points to the substrate or the geometry, while one that moves around points to contamination or the line. Film thickness, cure and adhesion checks then separate the remaining causes.

Work through the lot in this order before writing a rejection:

1. Map the defect. Mark where it occurs on several parts. Same spot every time, on thick sections, machined faces of castings or near welds: suspect trapped gas. Random spots, clusters on one rack position or one date code: suspect contamination or line conditions. Only on outer edges: suspect sharp edges and thin film.
2. For craters, trace the contaminant. Ask the coater when the compressed-air filters and moisture separators were last serviced, whether blast media is reused, and whether another powder ran in the booth just before your parts; on your side, check whether the parts were sprayed with a silicone or penetrating-oil product, or packed in oiled paper, before they shipped to finishing. A crater pattern that changes with the coating date points to the line; one that follows a machining batch points to the parts.
3. Look closely. Use a low-power magnifier. A small round hole with a raised rim, recurring in the same place on each part, is usually gas; a wide, shallow, saucer-shaped depression is usually a fish eye; a raised speck is dirt.
4. Measure film thickness. Take readings at the defect area and at agreed reference points per ASTM D7091, which covers magnetic gauges on steel and eddy-current gauges on aluminum (ASTM D7091-22). Very high readings near pinholes or orange peel point to excessive build; very low readings at edges confirm thin coverage.
5. Check whether holes reach the metal. On faces where corrosion protection matters, a low-voltage wet-sponge holiday detector finds discontinuities that the eye misses; per ASTM D5162, Method A is generally used for coatings 0.5 mm (20 mil) thick or less, and the allowable number of discontinuities should be decided before the test (ASTM D5162-24).
6. Check cure. A solvent rub test per ASTM D5402 is a quick way to separate an under-cured film from a well-cured one; the practice does not set the solvent, rub count or pass level, so take those from the powder supplier’s data sheet (ASTM D5402-19). If the rub test fails, ask the coater for a part-temperature record from the oven for that lot: heavy, thick-walled parts need longer dwell than light-gauge parts to reach cure temperature, so an oven set-point alone does not prove the part was cured.
7. Check adhesion. If parts are chipping or flaking, run a cross-cut tape test away from the defect; per ASTM D3359, Method B is for coating systems less than 125 µm (5 mil) in total thickness and Method A for thicker ones, so measure the film first, since many single-coat powder films fall under that limit but heavy builds and two-coat systems may not (ASTM D3359-23). Clean metal under the removed film points to pretreatment; failure only on machined faces points to coolant residue.
8. Check the packaging. Chips that line up with straps, dividers or part-to-part contact are transit damage, which is a packing instruction problem, not a coating problem.

Keep the evidence: photographs with a scale, the part numbers and lot or date code, thickness readings and test results. That record is what lets the supplier trace the defect to a line, a rack or a casting cavity, and it is the core of any 8D corrective action report you ask for.

What should the drawing, RFQ and purchase order say so the same defects do not come back?

Write down the surface classes, the viewing conditions, the allowed count of each defect per surface, the film thickness range and where it is measured, the adhesion and cure tests, and the edge condition you need before coating. Then approve a limit sample at first article so both sides judge the same thing.

• Surface classes. Mark significant (cosmetic) surfaces and functional surfaces on the drawing; everything else is non-significant. Inspectors cannot reject against a class that was never defined.
• Viewing conditions. State the viewing distance, angle, lighting and inspection time for each class. Typical industrial practice is to inspect cosmetic faces at a fixed distance under diffuse light with a limited time per face, so that a defect a trained eye cannot find under those conditions is not grounds for rejection.
• Defect counts. For each class, give the maximum number and size of pinholes, craters and inclusions per face, and state that blisters, flaking and bare metal are not permitted on functional surfaces.
• Film thickness. Give a minimum and maximum, the measurement method per ASTM D7091, and the number and location of readings, as explained in the powder coat thickness guide. A single nominal value invites both thin edges and fit problems.
• Adhesion and cure. Name the ASTM D3359 method and minimum class, and a cure check such as a solvent rub per ASTM D5402 with the supplier’s rub count. The adhesion guide covers how to pick the grade.
• Edges and masking. Require a broken or radiused edge on coated edges that need protection, because a knife edge cannot hold film; name masked threads, bores and grounding points as in the masking guide; and say where hang points and contact marks may go.
• Substrate notes. For die castings, galvanized steel or weldments, ask for a degassing pre-bake on the routing. For machined parts, require cleaning to remove coolant before pretreatment.
• Packing. Specify separators or individual wrapping for cosmetic parts, since chips from part-to-part contact are the cheapest defect to prevent.

A finish note can then read, for example: “Powder coat, color and gloss per note 5. Film thickness per ASTM D7091 at points A to D, minimum and maximum per table. Adhesion per ASTM D3359 Method B, class 4B minimum. Surfaces marked S: no pinholes, craters, blisters or inclusions visible at the stated viewing conditions; limit sample approved at first article. Edges marked E: break all edges before coating. No touch-up on surfaces marked S.” Adjust each value to your product; the methods come from the standards, but the limits are yours to set.