Surface Finishes Guide
As-machined, bead blast, anodize, powder, plating, painting and polishing
Surface finish serves two masters: function (corrosion, wear, friction, hygiene) and appearance. Options span as-machined (Ra 1.6–3.2 µm, free), blasting (uniform matte), anodizing (aluminum, colored or hard), plating (zinc/nickel), powder and paint (cosmetic + protection), and polishing (Ra 0.2–0.4 µm). Cost scales with process steps and area — specifying the minimum finish that meets function is a real price lever.
1. Mechanical finishes
As-machined (Ra 1.6–3.2 µm) is the default and free; visible tool marks are normal. Bead/sand blasting (Ra 0.8–1.6 µm) homogenizes the surface and hides marks at small cost — the most common upgrade on CNC parts. Polishing (Ra 0.2–0.4 µm) serves optics, sealing faces and cosmetics at real cost.
Shot peening compresses the surface for fatigue (springs, axles); grinding/lapping correct geometry to ±0.01 mm. For printed parts, media blasting is near-standard; vapor smoothing (FDM) and polishing (SLA) upgrade surfaces.
2. Chemical and coating finishes
Anodize (aluminum): Type II 5–25 µm, decorative, dyed any color; Type III hard coat 25–75 µm for wear; chromate conversion under paint. Plating (steel/brass): zinc 5–12 µm (corrosion), nickel (hardness), electroless nickel (uniform coverage), black oxide (cosmetic, thin).
Passivation (stainless) restores the chromium oxide layer and removes embedded iron — specify for food, medical and corrosion duty. Powder coat (25–100 µm) and paint give color and protection on steel/aluminum; specify the cosmetic face and coating standard.
3. Choosing by function
Corrosion outdoors → anodize (Al) or zinc + paint (steel); marine → 316 or hard anodize; wear → hard anodize, nickel, or DLC/carbide coatings; friction → PTFE/nickel composite; hygiene → electropolished stainless; appearance → dyed anodize or powder in brand color.
Match spec to environment: a salt-spray rating (ASTM B117 hours) is the objective target for corrosion claims; specify hours rather than vague “good corrosion resistance”.
4. Cost and drawing notes
Finish costs by area and steps: blast +$2–8, anodize +$3–10, powder +$5–15 per part typical on small parts; polishing and special coatings cost more. Mark the finish spec on the drawing (type, class, thickness, color) and the cosmetic face — unmarked faces finish as-machined.
Combine operations in one vendor pass (anodize + laser mark) to save handling; avoid plating after welding (trapped chemistry) and specify masking for threaded holes and datums.
Quick reference
| Finish | Typical Ra / thickness | Cost | Best for |
|---|---|---|---|
| As-machined | 1.6–3.2 µm | Free | Default, hidden faces |
| Bead blast | 0.8–1.6 µm | Low | Uniform matte look |
| Anodize II | 5–25 µm | Low-med | Aluminum color/protection |
| Hard anodize III | 25–75 µm | Med | Wear surfaces |
| Zinc plate | 5–12 µm | Low | Steel corrosion |
| Electroless Ni | 5–25 µm | Med | Uniform, hardness |
| Powder coat | 25–100 µm | Med | Color + durability |
| Polish | 0.2–0.4 µm | High | Optics, seals |
Sources & references
- MIL-A-8625 — Anodize
- ASTM B117 / ISO 9227 — Salt spray
- Xometry Learning Center — Surface Finishes (public engineering reference)