Powder coating & anodizing
Will the twelfth batch match the sample you approved in March?
Finish is the part of an aluminium order that fails latest and loudest — usually on site, after installation, when nobody can rework it. The cause is almost never the coating itself. It is a colour that drifted between paint lots, a film that went thin on an edge, or a machining cut made after anodizing. This page is how each of those is controlled, and the wording to put in your purchase order so it is enforceable.
Choosing
What each finish actually buys you, and where it stops
Buyers often choose between powder and anodizing on appearance. The decision that matters is exposure: anodizing is a conversion of the metal surface, powder is a film on top of it. They fail in different ways, and they fail at different thicknesses.
| Finish | Film | Standard | Salt spray | Use it for | Where it stops working |
|---|---|---|---|---|---|
| Powder coatAkzoNobel / TIGER | 60–120 µm (80 nominal) | Qualicoat Class 1 AAMA 2604 | 1,000 h | Exterior frames, machine guarding, colour-critical batches, any RAL | Edges and sharp corners, where the film thins. Also anything that gets machined after coating. |
| Anodize, Type IISulphuric | 10–25 µm (15 nominal) | AAMA 611 ISO 7599 | Seal-dependent | Architectural interiors, clear and bronze tones, wear surfaces, parts that will be handled | Colour shift between 6063 and 6061 on the same rack. Deep tones are hardest to hold across lots. |
| Anodize, architectural25 µm class | 25 µm | AAMA 611 | Higher, sealed | Exterior façade where the anodized look is specified | Cost, and lead time — it is a slower pass through the line. Not available on every alloy. |
| Mill finishAs extruded | None | — | — | Parts that will be machined, or finished by your own line | Handling marks and die lines are visible and permanent. Not a cost saving if the part is seen. |
| Wood grainSublimation | 80–120 µm | Qualicoat base | 1,000 h base | Interior decoration, furniture profiles, louvres | UV fade on south-facing exterior use. The base coat performs; the pattern does not. |
The batch problem
Four control points between one batch and the next
Colour drift is not a mystery and it is not random. It comes from four variables, and each one has a control that either exists at the supplier or does not. Ask which of these four are in place before you approve a colour.
Approved sample, retained
One physical sample per colour, signed and dated, kept at the line — not a photo and not a Pantone chip on a screen.
Single paint lot
Paint is ordered per project lot where volume allows. Mixing two lots mid-order is the most common cause of a visible seam.
ΔE measurement
Measured against the retained sample on every lot, with a stated limit. “Looks the same” is not a measurement.
Film thickness per face
Five readings per length, minimum as well as nominal. Averages hide the thin edge that will fail first.
Practical
Put this in the purchase order, not in an email
Most finish disputes are not about capability — they are about a specification that never named a standard, a thickness or a tolerance. This is the clause we work to. Copy it, change the colour and the numbers, and send it with the PO.
Surface finish: polyester powder coating to Qualicoat Class 1 / AAMA 2604. Colour: RAL 7016, matt (gloss unit 20–30 at 60°). Film thickness: 80 µm nominal, 60 µm minimum on all visible faces, measured to ISO 2360 at five points per length. Colour: measured against the approved sample retained by the supplier and dated at first approval. ΔE ≤ 1.5 (D65, 10° observer). Adhesion: cross-cut Gt0 to ISO 2409 on a production part, not a test panel. Corrosion: 1,000 h neutral salt spray to ISO 9227, no blistering beyond rating 8 and no penetration at scribe beyond 2 mm. Edges and machined faces to be fully covered; no bare aluminium permitted on any face visible after assembly.
Failure modes
Four ways a finish fails, and which are preventable at the drawing stage
Thin film on an edge
Powder pulls back from sharp corners, so the minimum lands on the edge that takes the impact. A 0.5 mm radius on the drawing fixes more of this than any process change.
Bare metal after machining
Drilling or milling after anodizing exposes raw aluminium at every cut. It fails a salt-spray review and it is visible. Sequence the machining before finishing, or specify the machined faces as mill finish from the start.
Colour shift between alloys
Anodizing on 6061 does not match 6063 in the same tank. If one assembly mixes both alloys, expect a visible difference and specify a colour tolerance that acknowledges it.
Outgassing pinholes
Trapped air in a porous casting or a contaminated extrusion escapes through the powder in the oven. It shows as a pinprick crater. Prevented by pre-treatment, not by a second coat.
Send the colour reference and the exposure conditions. We will tell you which finish holds and which will not.
Ask about a finish →