The finish is what customers see and touch — and a frequent source of rejected parts: color that differs between batches, bearing bores that no longer fit after coating, or a grounding surface that is suddenly insulated. Choosing and specifying the finish correctly avoids all three.
Common finishes compared
| Finish | What it does | Typical thickness | Conductive? | Good for |
|---|---|---|---|---|
| Anodizing Type II (sulfuric) | Grows an oxide layer; can be dyed | ≈ 5–25 µm | No | Cosmetic colored parts, corrosion protection |
| Hard anodizing Type III | Thicker, very hard oxide | ≈ 25–75 µm | No | Wear surfaces, sliding parts |
| Chromate conversion (e.g. “Alodine”) | Thin conversion coating | Very thin (sub-micron) | Yes | Electrical grounding, paint base |
| Powder coating | Baked polymer coating | ≈ 60–120 µm | No | Robust colored finish on sheet metal and enclosures |
| Bead blasting | Uniform matte texture | — | — | Pre-treatment before anodizing; hides tool marks |
| Brushing / polishing | Directional or reflective surface | — | — | Premium consumer products |
Thickness ranges are typical; specify the requirement and the standard (for example MIL-A-8625 types for anodizing, MIL-DTL-5541 for chromate conversion) on the drawing.
Finishes change dimensions
Anodizing converts the surface: roughly half the coating thickness builds outward and half penetrates the metal. Holes shrink, shafts grow, and threads can bind — especially with hard anodizing. Powder coating adds far more. For precision features choose one of three approaches and state it on the drawing:
- Mask the feature (bores, threads, mating faces).
- Machine with allowance for the coating build-up.
- Machine after coating where the bare surface is acceptable.
Color consistency
- Dyed anodizing color depends on the alloy, surface preparation and bath conditions — 7075 and cast alloys usually anodize darker and less uniformly than 6061 or 6063.
- Parts made from different alloys or different material batches may not match side by side.
- Approve a color sample (or a range of limit samples) and keep it as the reference.
- Anodize mating cosmetic parts in the same batch when appearance matters.
- For exact color matching across materials, powder coating or painting is usually easier than anodizing.
Electrical and functional requirements
Anodizing and powder coating are insulators. Grounding points, EMI gasket contact areas and connector interfaces need masking or a conductive finish such as chromate conversion. Call these out explicitly — a missing ground contact is a common cause of EMC test failures in metal enclosures.
How to specify a finish on the drawing
- Finish type and standard (e.g. “Anodize per MIL-A-8625 Type II, Class 2, black”)
- Color reference and gloss level, or approved sample ID
- Cosmetic surfaces (A / B / C) and acceptable defects
- Masked areas and dimensions that apply after finishing
- Pre-treatment (bead blast media and grit, brushing direction)
See also choosing aluminum, steel or stainless and our CNC machining finishing options — or send your drawing for a finish recommendation.