Reviewed and updated September 12, 2026
What anodizing changes
The coating is formed from the aluminum surface rather than simply deposited on top like paint. Part of the oxide grows outward and part inward, so finish thickness can affect tight fits, bores, threads and electrical contact points. The amount depends on the specified process; confirm dimensional strategy with the supplier rather than applying a universal allowance.
Why color and gloss vary
Alloy composition, temper, billet history, surface finish, pretreatment, bath control, dye and sealing can all influence appearance. Chemical etching or blasting can produce a matte base before anodizing, while machining marks may remain visible under a clearer finish.
| Requirement | Communicate on the drawing or PO |
|---|---|
| Color-critical enclosure | Color reference, acceptable range, cosmetic zones and approved sample if available |
| Mating bore or bearing seat | Final size requirement and whether it applies before or after finish |
| Ground or electrical contact | Explicit masking location and permitted boundary |
| Threaded feature | Masking/tapping expectation and final gauge requirement |
| Uniform matte look | Pretreatment method and acceptance sample |
Common process families
International drawings often reference sulfuric-acid anodizing and distinguish conventional decorative/protective finishes from thicker hard anodic coatings. Names such as “Type II” and “Type III” are commonly encountered in North American supply chains, but a label alone is incomplete. State the governing standard and revision, class or color, thickness where required, sealing, masking and any functional tests.
Design and RFQ checklist
- Specify the exact aluminum alloy and temper.
- Mark cosmetic surfaces and acceptable rack/contact locations.
- Identify threads, bores, datum targets and electrical contacts to mask.
- State whether dimensional limits apply before or after anodizing.
- Use a physical limit sample when color matching is business-critical.
- Define corrosion, wear or electrical requirements instead of relying only on a color name.
Questions to resolve before production
- Is the finish decorative, protective, wear-oriented or electrically functional?
- Will multiple parts be assembled edge-to-edge, making color variation more visible?
- Are mixed alloys or manufacturing lots involved?
- Which surfaces may show contact or rack marks?
- What inspection or certificate is required?
Sources and further reading
This guide summarizes general practice and avoids prescribing a coating specification without a part drawing and service environment.