Surface Finish Comparison Chart for CNC Parts
Nine finishes side by side — anodizing Type II and hardcoat, passivation, black oxide, zinc plating, electroless nickel, powder coating, bead blasting, polishing. Real thickness, hardness, salt spray hours, cost and lead time from our own supplier chain.
The Three Questions That Pick Your Finish
Before you look at the table, answer these. They eliminate most options immediately:
- What is the part made of? Anodizing only works on aluminum, titanium and magnesium. Passivation only works on stainless. Black oxide and zinc plating only work on ferrous metals. This alone usually leaves you with two or three candidates.
- Is the finish cosmetic or functional? If a customer will look at it, color consistency matters more than hardness. If it slides, wears or sits outdoors, the reverse is true.
- Does the part have tight fits? Every coating adds thickness. If you have an H7 bore or a sliding shaft, you must machine undersize/oversize to compensate, or mask the feature off.
The table below is what we hand to customers who ask "what finish should I use?" — sorted roughly by how often we run each one.
Nine Finishes Compared
Scroll right on mobile. Salt spray figures are neutral salt spray per ASTM B117 at the thickness shown — realistic, not marketing numbers.
| Finish | Typical thickness | Hardness | Salt spray (ASTM B117) | Color options | Applies to | Relative cost | Added lead time |
|---|---|---|---|---|---|---|---|
| Anodizing Type II | 5–25 μm | 200–300 HV | 300–600 h (sealed) | Clear, black, any dye color | Aluminum (best on 6061/6063) | 1.0× | 2–3 days |
| Anodizing Type III (hardcoat) | 25–100 μm | 400–600 HV | 500–1,000 h | Natural dark grey, black (limited dyes) | Aluminum, titanium | 1.6× | 3–5 days |
| Passivation (ASTM A967) | None (removes ~0.001mm) | Unchanged | 500–1,000+ h | None — bare metal look | Stainless steel only | 0.8× | 1–2 days |
| Black oxide | ~1 μm | Unchanged | 12–48 h (needs oil/sealer) | Black only | Carbon & alloy steel | 0.6× | 1–2 days |
| Zinc plating (clear/blue/yellow) | 5–25 μm | ~70 HV (soft) | 96–500 h (by chromate) | Clear, blue, yellow, black | Steel, iron | 0.9× | 2–4 days |
| Electroless nickel (ENP) | 5–50 μm, uniform | 500–700 HV (1,000 HV heat-treated) | 500–1,000+ h | Satin silver/grey | Steel, aluminum, copper, plastics | 2.5× | 4–7 days |
| Powder coating | 60–120 μm | 2H–4H pencil | 500–1,000 h (with pretreatment) | Full RAL range, textures, gloss levels | Aluminum, steel (conductive) | 1.4× | 3–6 days |
| Bead blasting | None (surface texture only) | Unchanged | No improvement alone | Matte grey, uniform grain | All metals, some plastics | 0.5× | 1 day |
| Polishing | None (removes material) | Unchanged | Slight improvement | Mirror to satin | All metals | 1.5–3× (labor heavy) | 2–5 days |
Cost relative to Type II anodizing on the same part size. Lead time is added on top of machining time. Salt spray hours vary with coating thickness, sealing quality and substrate — treat as typical, not guaranteed.
Which Finish for Which Job
| If you need… | Use this | Why not the alternatives |
|---|---|---|
| A colored aluminum enclosure customers will see | Anodizing Type II, dyed | Powder coat hides machined detail and adds 100 μm; Type III can't take bright colors |
| An aluminum part that slides or wears | Anodizing Type III (hardcoat) | Type II wears through in weeks; hardcoat gives 2–3× the surface hardness |
| Stainless steel that must not rust | Passivation (ASTM A967) | Plating on stainless is usually unnecessary and can flake; passivation is cheaper and adds zero thickness |
| A black steel part, indoor use, lowest cost | Black oxide + oil seal | Only 12–48 h salt spray — fine indoors, will rust outdoors. Zinc or ENP if it goes outside |
| Steel fasteners or brackets for outdoor use | Zinc plating with yellow or black chromate | 96 h clear / 200 h yellow / 500 h black chromate; cheap and standardized |
| Uniform coating inside bores and blind holes | Electroless nickel (ENP) | Electroplating throws unevenly — inside corners get thin. ENP is electroless, so thickness is uniform everywhere |
| A large aluminum housing, thick durable color | Powder coating | Anodizing gets expensive on large parts; powder coat is cheaper per dm² and more impact resistant |
| Remove tool marks, matte uniform look | Bead blasting (glass bead #80–120) | Cheapest cosmetic option; combine with anodize or clear coat for protection |
| Mirror finish for optical or food-contact | Mechanical polishing, then electropolish if stainless | Labor-intensive and the most expensive per part; only worth it where specified |
Coating Thickness vs. Your Tolerances
This is the mistake that costs the most money and the most arguments. Every coating adds material to the surface. If you machine a 10mm shaft and then hardcoat anodize it at 50 μm, the shaft is now 10.05mm — and it will not fit the 10mm bearing you designed for.
The rules we work to:
- Anodizing — roughly half the coating thickness builds outward per surface. A 20 μm Type II coating adds about 10 μm per side. For a bore, the hole gets smaller by 20 μm on diameter.
- Electroless nickel — builds up the full nominal thickness per surface. A 25 μm ENP coating adds 50 μm to a shaft diameter.
- Powder coating — 60–120 μm per surface. On a mating face this is enormous; mask it or machine for it.
- Passivation, black oxide, bead blast — negligible dimensional change. Safe for tight fits.
What we recommend: either mark the drawing "dimensions apply after finishing" and let us compensate in machining, or call out which features must be masked. Masking adds cost per part but is far cheaper than a scrapped batch.
One more that catches people: threads. Anodizing or plating a threaded hole will change the fit class. For critical threads, either mask them, use a slightly oversized tap, or specify the thread be chased after coating. Tell us which on the drawing and it's a non-issue.
Finishing Questions We Get Every Week
What is the difference between Type II and Type III anodizing?
How much does anodizing add to part dimensions?
Which surface finish has the best corrosion resistance?
Can you anodize 7075 aluminum?
Is powder coating or anodizing better for aluminum?
Do I need to passivate stainless steel?
Send the Drawing — We'll Specify the Finish With You
Finishing is where a lot of parts go wrong — wrong coating for the environment, or a coating that breaks a fit. Send us the drawing with your service environment and we'll recommend a finish, flag any dimensions that need masking or compensation, and quote it as one line item alongside machining.