You've designed the part, chosen the material, dialed in the tolerances. The last box on the RFQ says "Surface Finish" — and that's where I see a lot of engineers freeze up and either leave it blank (bad) or write something unnecessarily expensive (also bad).

Surface finish affects how your part looks, resists corrosion, wears against mating surfaces, and seals. Here's what each finish actually is, what it costs, and when you should pick it.

The roughness numbers that matter

Ra (Roughness Average) is the go-to metric. Lower Ra = smoother. For CNC parts:

Finish Ra Range Typical Use
Rough machined Ra 3.2–6.3 μm Clearance surfaces, non-critical brackets
Standard machined Ra 0.8–3.2 μm Most general-purpose parts
Fine machined Ra 0.4–0.8 μm Sealing surfaces, bearing bores
Polished Ra 0.1–0.4 μm Optical surfaces, hygienic apps
Mirror Ra < 0.1 μm Decorative, reflective

For reference: a sharp carbide end mill at standard parameters produces Ra 0.8–1.6 μm — smooth enough for most functional surfaces with no additional processing.

1. As-machined

The surface left directly by the cutting tool. Tool marks are normal and expected.

Appearance: Parallel tool path lines, matte metallic. Facing = circular pattern. Side milling = linear ridges at the step-over interval. Ra: 0.8–3.2 μm standard, 0.4–1.6 μm with fine finish pass. Cost: No extra — this is the default.

Use when: Internal non-visible surfaces, functional surfaces where tool marks don't hurt performance, parts getting painted/coated (texture improves adhesion), cost-sensitive work.

Skip when: Cosmetic external surfaces on consumer products, sealing surfaces needing Ra < 0.8 μm, food/medical where tool marks trap contaminants.

If you want a cleaner as-machined look, specify reduced step-over on finish pass (0.05mm instead of 0.10mm). Adds some cycle time but produces noticeably finer tool marks.

2. Bead blasting

Glass or ceramic beads blasted at controlled pressure. Peens the surface, removes tool marks, uniform matte texture.

Appearance: Satin matte, consistent light-scattering. Light to medium gray on aluminum. Tool marks completely gone. Ra: 1.6–3.2 μm cosmetic, 0.8–1.6 μm fine. Cost: Low to moderate. Budget $3-15/part for small-medium components. Masking threaded holes adds labor.

Use when: Consumer-facing surfaces needing clean matte look, prep for anodizing (bead blast + anodize is classic), parts handled frequently (texture hides fingerprints).

Skip when: Tight-tolerance sealing surfaces (it removes material), threaded holes without masking, optical/mirror requirements.

Important: specify "bead blast all external surfaces, mask threaded holes and bores." If you don't say mask, assume the shop blasts everything.

3. Anodizing (Type II and Type III)

Electrochemical conversion of aluminum surface into hard aluminum oxide. Type II = decorative (thin, dyeable). Type III = hard anodize (thick, wear-resistant).

Appearance: Type II — uniform color, satin sheen. Available in clear, black, red, blue, gold, custom. Type III — dark gray to charcoal, matte. Limited colors.

Thickness: Type II 5–25 μm. Type III 25–100 μm. The layer adds to dimensions — design for it. Cost: Moderate. Type II clear/black is commodity pricing. Custom colors cost more. Type III more expensive.

Materials: Aluminum only. 6061 anodizes beautifully. 7075 gets a yellowish tint (copper content). Hard anodize preferred for 7075.

Use Type II for: Consumer electronics, automotive trim, general aluminum corrosion protection.

Use Type III for: Wear surfaces, aerospace structural, marine hardware, max surface hardness.

Skip for: Parts with pressed-in hardware (hole diameters change), tight-tolerance external features (hard anodize adds measurable thickness).

If your part gets anodized, add: "Mask threaded holes and tight-tolerance bores." The anodize layer adds ~50% of its thickness to the surface — a 25μm layer grows a hole 0.05mm smaller.

4. Alodine (Chromate Conversion Coating)

Chemical conversion for corrosion protection without thickness. Electrically conductive.

Appearance: Light gold to tan, slightly iridescent. Does not hide tool marks. Thickness: < 1 μm. Cost: Low.

Use when: Aerospace parts needing conductivity, parts where anodizing thickness is unacceptable, primer before painting, cost-sensitive corrosion protection.

Skip when: Decorative parts (industrial-looking), high-wear surfaces, frequently handled parts (can rub off).

5. Powder coating

Dry polymer powder electrostatically applied and oven-cured. Thick, durable coating.

Appearance: Uniform opaque color, slight orange-peel texture. Any color, any gloss. Thickness: 50–150 μm. Cost: Moderate. Per-part drops sharply with quantity.

Use when: Industrial enclosures, brackets needing durable color, weather-exposed parts, hiding surface imperfections.

Skip when: Precision-fit surfaces, threaded holes without masking, tight flatness requirements (cure temp can warp parts), applications above ~120-150°C.

6. Brushing and polishing

Mechanical abrasive treatment. Brushing = directional grain. Polishing = reflective.

Appearance: Brushed — linear grain, satin sheen. Polished — progressively brighter. Ra: Brushed 0.4–1.6 μm. Polish 0.1–0.4 μm. Mirror < 0.1 μm. Cost: Brushing moderate, polishing expensive (labor-intensive).

Use brushing for: Cosmetic covers, architectural components.

Use polishing for: Optical surfaces, hygienic/food-contact, sealing surfaces Ra < 0.4 μm, high-end decorative.

Skip for: Internal surfaces, complex 3D contours (inaccessible to polishing), high-volume production (doesn't scale).

7. Passivation (stainless steel)

Chemical treatment removing free iron, promoting chromium oxide passive layer. Not a coating.

Appearance: Invisible. Cost: Low.

Use when: ALWAYS on stainless parts — untreated stainless has free iron from machining that rusts. Medical devices, food processing, any stainless seeing moisture.

Limitations: Not a substitute for material selection. 303 stainless rusts in marine environments passivated or not — use 316.

How I think about choosing

  1. Is the surface visible? No → As-machined.
  2. Needs corrosion protection? No → Bead blast or brush for looks.
  3. What material? Aluminum → Anodize or alodine. Stainless → Passivation. Other → Paint or powder coat.
  4. Needs wear resistance? → Type III hard anodize (aluminum).
  5. Sealing or bearing surface? → Specify the Ra, let shop determine method.

What to write on your RFQ

Be specific. More specific = more accurate quote = fewer surprises:

Surface finish is functional, not decorative. For most parts: as-machined with fine finish pass. For parts facing elements: anodize or alodine. For sliding parts: hard anodize or polish. For needing looks AND function: bead blast + anodize.