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Anodize or powder coat? Choosing a finish for machined aluminium parts
Published 2026-07-23
For machined aluminium, nine finish decisions out of ten come down to two lines: anodizing or powder coating. They are not a better-and-worse pair but two entirely different ways of building a surface — choose well and the finish becomes part of the part's engineering; choose badly and, at best, the look is off, at worst the fit no longer assembles. Here is a decision order you can apply directly.
Start with the physics: a film that grows, versus a film that covers
Anodizing is not a coating. It converts the aluminium surface itself into a dense oxide film — the film grows out of the metal, typically 8–30 µm, roughly half inward and half outward from the original surface. It keeps the metallic look, cannot peel in the way paint peels, and offers clear, black and a limited dye palette.
Powder coating electrostatically deposits a polymer powder that is then baked into a continuous paint film, typically 60–120 µm, laid over the metal. Its strength is colour: the full RAL range, matte to gloss, textures — a uniform solid surface under which the metallic character disappears.
Walk five axes and the answer usually surfaces on its own
Run your part down the table below. In most cases the direction is settled by the third row.
| Axis | Anodizing | Powder coating |
|---|---|---|
| Appearance | Metallic character; clear, black, limited dyes | Solid colour; full RAL range, matte/gloss/texture |
| Film build & fits | 8–30 µm — small but never negligible on close fits | 60–120 µm — mating faces must be masked or post-machined |
| Outdoor exposure | Stable film, keeps the metallic look long-term | Excellent colour weathering; the architectural standard |
| Hardness & wear | Hard oxide, scratch-resistant | Tough, impact-tolerant film, softer than oxide |
| Conductivity | Insulating — grounding points need masking to bare metal | Equally insulating — mask grounding and shielding faces |
The order question: machine first, or finish first?
The default is machine first, finish second: cut every feature, then send the part down the line so the film covers every face and no cut edge shows bare. The reverse — finish first, machine after — belongs to specific cases, such as anodized stock cut to length later, where the cut face shows base metal: acceptable indoors, rarely acceptable on visible or outdoor parts.
What genuinely needs deciding early is masking. Grounding faces that must conduct, precision bores that must fit, and every thread need an explicit keep-bare call before the part enters the tank. Film grown into a thread means a stiff assembly at best and rework at worst — marking masked areas on the drawing is the cheapest line of text in the whole finishing chapter.
Why heat sinks so often run black anodize
Heat sinks deserve their own paragraph. A black anodic film raises surface emissivity, which measurably helps radiation-dominated, natural-convection cooling — while also unifying the look and resisting fingerprints and scratches. A powder film, far thicker, does the opposite for heat: it adds a resistive blanket. So the standard heat-sink recipe reads: mounting face kept bare (or masked and post-machined) for contact conduction, everything else black anodized.
When mill finish is the right answer
Not every part needs treatment at all. Structural parts inside an enclosure, components the customer will paint at machine level, short-life fixtures — machined bare aluminium serves perfectly well, and saves a process step along with its lead time. The test is three questions: will this face be seen, will it be touched, what will it be exposed to? Three no's, and mill finish is the engineering answer, not a compromise.
When you do specify a finish in an RFQ, put four things on one line: treatment, colour (RAL reference or a physical sample), film class, masked areas. Those four settled, the finishing chapter of your enquiry will not generate a single follow-up question.