SURFACE FINISHING
Surface Finishing Options for CNC Machined Parts
Pick a finish by the problem you need to solve — corrosion, wear, colour, electrical contact, or a tight fit that must not grow. Every card below links to the process page and tells you the Ra, the film thickness and the materials that finish will accept.
Start from the problem
START HERE
How to choose a surface finish: start from the problem
Six decision cards. Each one names the finishes that answer it and the reason why, then links straight into the process page.
Corrosion protection
The part lives outdoors or in moisture, and rust is the failure mode. These finishes build a barrier or a conversion layer on the surface itself.
Wear & abrasion resistance
It rubs, slides or gets scratched, and you need it to keep its shape. These raise surface hardness rather than just covering the part.
Colour & appearance
The finish is the product — colour, gloss or texture sells it. These are the finishes with a real colour or texture range.
Electrical contact & insulation
It has to stay conductive — or the opposite, it has to insulate. Contact resistance decides which of these you can use.
No dimensional change
The fit is tight and the finished part still has to measure up. These add essentially no dimension — everything in the plating table does.
Food, skin & medical contact
It touches food, skin or a patient, and has to clean down. Smooth, passive and non-shedding surfaces — no dye, no porous coating.
REFERENCE
Surface finish chart: Ra, thickness and materials by process
The column that most drawings argue about is Ra achievable — it is the number engineers search for, and it is what decides whether a finish is even on the table.
Ra is the arithmetic mean roughness of the finished surface, in micrometres. Thickness is the added layer — “—” means the process removes or reshapes material instead of adding any. Cost is relative to an as-machined part: $ baseline, $$$$ the most expensive. Figures are the published process range for each finish — where your drawing sets a tighter limit, the drawing wins, and we confirm what we can hold on the quote.
| Finish | Ra achievable (µm) | Thickness (µm) | Materials | Colour & appearance | Best for |
|---|---|---|---|---|---|
| Mechanical finishingChanges the surface geometry. Removes material or textures it in place — adds no coating. | |||||
| As-machined$ | 3.2 – 6.3 | — | Metals, plastics | Tool marks visible | Functional faces, no cosmetic requirement |
| Smooth machining$$ | 0.8 – 1.6 | — | Metals, plastics | Smooth | Cosmetic metal with a fine cut |
| Fine machining$$$ | 0.8 | — | Metals, plastics | Smooth | Sealing faces, bearing bores |
| Sanding$ | 0.8 – 3.2 | — | Metals, plastics | Matte, uniform | Knocking back marks before polishing |
| Brushing$ | 0.8 – 1.6 | — | ABS, aluminium, brass, stainless, steel | Satin, directional | Decorative metal, hides fingerprints |
| Polishing$$$ | 0.2 – 0.8 | — | Aluminium, brass, stainless, steel | Glossy to mirror | Mirror cosmetics, low friction |
| Bead blasting$ | 1.6 – 3.2 | — | ABS, aluminium, brass, stainless, steel | Matte, uniform | Pre-paint prep, uniform matte look |
| Chemical conversionReacts with the base metal to grow a film. Adds almost nothing — usually under 1 µm. | |||||
| Passivation$ | 1.6 – 3.2 | under 1 | Stainless steel | Unchanged, matte | Free-iron removal, zero dimensional change |
| Alodine (chem film)$ | 1.6 – 3.2 | 0.25 – 1.0 | Aluminium | Clear, gold | Conductivity plus a paint base on aluminium |
| Yellow chromate$ | 1.6 – 3.2 | 0.25 – 1.0 | Aluminium, steel | Yellow, iridescent | Post-plating passivation, corrosion and paint base |
| Black oxide$ | 1.6 – 3.2 | under 1 | Steel, stainless steel | Black, matte | Tooling, light corrosion protection |
| Anodizing Type II$$ | 0.8 – 3.2 | 5 – 25 | Aluminium | Clear, black, grey, red, blue, gold | Corrosion plus colour on aluminium |
| Hard anodizing Type III$$$ | 0.8 – 3.2 | 25 – 75 | Aluminium | Dark grey, black | Wear and abrasion on aluminium |
| Plating & coatingDeposits an added layer. Thickness is real and must be designed around. | |||||
| Zinc plating$ | 1.6 – 3.2 | 3 – 12 | Steel, stainless steel | Bright blue, black or silver | Steel corrosion protection |
| Nickel plating$$ | 0.8 – 1.6 | 3 – 12 | Aluminium, steel, stainless | Clear, matte, black | Wear plus corrosion on steel |
| Electroless nickel$$$ | 1.6 | 5 – 8 | Aluminium, steel, stainless | Smooth, glossy | Even coating on complex shapes |
| Electroplating$$ | 0.4 – 1.6 | 1 – 25 | Aluminium, steel, stainless | Metallic, glossy | Conductivity plus wear resistance |
| Powder coating$$ | 2.0 – 5.0 | 60 – 100 | Aluminium, stainless, steel | Black or white, 20% / 90% gloss | Outdoor use, covering surface defects |
| PTFE (Teflon) coating$$ | 1.6 – 3.2 | 15 – 30 | Steel, aluminium | Black, smooth | Low friction, chemical resistance |
| Laser cladding$$$$ | 6.3 – 12.5 | 250 – 2000 | Steel | Weld overlay, machinable after | Repair and wear-resistant overlay |
DECIDE
Surface finish comparison matrix: corrosion, wear and cost
Read down a column to see which finishes are strong at one property, or read along a row to see where a single finish suits your base material.
| Finish | Corrosion | Wear | Appearance | Conductive | No size change | Aluminium | Stainless | Carbon steel | Plastics | Cost |
|---|---|---|---|---|---|---|---|---|---|---|
| Anodizing Type II | ● | ◐ | ● | ○ | ◐ | ● | ○ | ○ | ○ | $$ |
| Hard anodizing Type III | ● | ● | ◐ | ○ | ○ | ● | ○ | ○ | ○ | $$$ |
| Alodine (chem film) | ◐ | ○ | ○ | ● | ● | ● | ○ | ○ | ○ | $ |
| Yellow chromate | ● | ○ | ◐ | ● | ● | ◐ | ◐ | ◐ | ○ | $ |
| Black oxide | ◐ | ◐ | ● | ○ | ● | ○ | ● | ● | ○ | $ |
| Passivation | ● | ○ | ○ | ◐ | ● | ○ | ● | ○ | ○ | $ |
| Zinc plating | ● | ◐ | ◐ | ● | ○ | ○ | ◐ | ● | ○ | $ |
| Nickel plating | ● | ● | ◐ | ● | ○ | ◐ | ◐ | ● | ○ | $$ |
| Electroless nickel | ● | ● | ◐ | ● | ○ | ● | ● | ● | ○ | $$$ |
| Electroplating | ● | ● | ● | ● | ○ | ◐ | ◐ | ● | ○ | $$ |
| Powder coating | ● | ◐ | ● | ○ | ○ | ● | ● | ● | ○ | $$ |
| PTFE (Teflon) | ● | ◐ | ◐ | ○ | ◐ | ◐ | ◐ | ● | ○ | $$ |
| Polishing | ○ | ○ | ● | ◐ | ◐ | ● | ● | ● | ● | $$$ |
| Brushing | ○ | ○ | ● | ◐ | ● | ● | ● | ● | ● | $ |
| Bead blasting | ○ | ○ | ◐ | ○ | ◐ | ● | ● | ● | ● | $ |
| Laser engraving | ○ | ○ | ◐ | ○ | ● | ● | ● | ● | ● | $ |
Conductivity and “no size change” are the two columns that most often remove a finish from consideration. Check them before you read the rest of the row.
ROUGHNESS
Surface roughness (Ra) chart: what each process can achieve
Ra is the arithmetic mean roughness of the surface profile. Lower is smoother, and every step down the list means one more operation on the part — which is why an unnecessary callout is the most expensive tolerance on a drawing.
Smoother than you need is not better. It is slower and it costs more.
The two rows marked quoted case by case sit at the far end of the range: they need a hand operation on a specific part, so we price them per drawing rather than from a table. Send the part and we will tell you what is achievable.
Read the full roughness reference →- Ra
12.5µm Rough milling, rough turning, sand casting Non-functional faces, cast or forged stock Open process → - Ra
6.3µm Standard milling and turning, laser cladding as-laid Structural parts, non-critical faces Open process → - Ra
3.2µm Standard finish cut, bead blasting, anodizing over a machined face General machined surfaces, brackets Open process → - Ra
1.6µm Fine turning, brushing, nickel plating, electroless nickel Mating faces, shafts, bushings Open process → - Ra
0.8µm Fine machining, fine grinding Sealing faces, bearing bores Open process → - Ra
0.4µm Precision grinding, honing Hydraulic spools, piston rods Quoted case by case → - Ra
0.2µm Lapping, mechanical polishing Bearing races, optical seating Open process → - Ra
0.05µm Superfinishing, lapping Gauge blocks, optical mirrors Quoted case by case →
BY MATERIAL
Surface finishing by material: aluminium, steel and plastics
The fastest way to shorten the list: two of these finishes will simply not work on your material.
- Aluminium (6061, 7075, 6082) Anodizing Type II for colour, Type III for wear, Alodine to stay conductive Not suitable: passivation, black oxide. Compounds vary — 6061 dyes far more evenly than 7075.
- Stainless steel (304, 316) Passivation as standard, bead blasting or brushing for appearance Not suitable: anodizing, alodine. Electropolishing if you need a hygienic finish.
- Carbon & alloy steel Zinc plating for outdoors, black oxide for tooling, nickel for wear Not suitable: anodizing, alodine. Powder coating only if a 60+ µm build-up is acceptable.
- Brass & copper Polishing or brushing for appearance, nickel plating for wear Not suitable: anodizing, black oxide, alodine.
- Titanium Anodizing for colour and for a stable oxide; polishing for cosmetics Not suitable: passivation in the stainless sense, zinc plating, alodine.
- Plastics (ABS, PC, nylon, POM) Bead blasting, sanding and brushing only — mechanical finishes Not suitable: every plating, coating and anodizing process here.
TOLERANCE
Coating thickness: what each finish adds to your part
Decide this before the drawing is released. If the finished part still has to fit, the film moves your dimensions — here is which way, and by how much.
Anodizing is the awkward one: it grows roughly half the film outward and half inward, so a bore and the shaft that goes into it both move. Everything in the plating group grows outward only.
| Finish | Typical film | Growth direction | What it means for the fit |
|---|---|---|---|
| Anodizing Type II | 5 – 25 µm | about 50% outward / 50% inward | A 20 µm film adds roughly 10 µm per side outward, and roughly 20 µm on diameter. |
| Hard anodizing Type III | 25 – 75 µm | about 50% outward / 50% inward | A 50 µm film grows about 25 µm per side — enough to close a tight bore or bind a thread. |
| Powder coating | 60 – 100 µm | all outward | A 60 µm build-up on both faces of a 120 mm gap removes 120 µm from the gap. |
| Zinc / nickel plating | 3 – 12 µm | all outward | Thin enough for most fits, but mask threads and dowel bores with an H7 callout. |
| Electroless nickel | 5 – 8 µm | all outward | Even on complex geometry, which is why it is chosen when a bore must stay true. |
| Passivation / Alodine | under 1 µm | effectively none | The only group here that can be specified on an already-finished, tight part. |
Surface finishing services, machined and finished under one roof
The finish is quoted with the part rather than bolted on afterwards. Tell us the surface requirement on the drawing and it is priced in the same quote as the machining, so a change of finish does not send the part to a second supplier.
GO DEEPER
Surface finishing guides: corrosion, wear and selection
Three ways in, depending on what you are trying to decide: how to stop corrosion, how to stop wear, and how to choose between two finishes that both look right.
Specs & charts
Start here if you already have a number to hit.
Pick between two finishes
The comparison questions our readers actually send.
Understand one process
Deep dives behind a single finish.
QUESTIONS
Surface finishing FAQ: the questions engineers ask us
What Ra value should I put on my drawing?
Only call a roughness where the surface does a job. Ra 3.2 µm is the usual machined default, Ra 1.6 µm for a mating face, Ra 0.8 µm for a sealing face, and below Ra 0.4 µm only for bearings, spools and optical work. Every step down adds an operation, so an unneeded callout is the most expensive line on the drawing.
Which finishes add no thickness at all?
Passivation, Alodine (chem film), bead blasting and brushing add essentially nothing — Alodine stays under about 1 µm. Everything in the plating and coating group does add thickness, and that has to be designed around, not assumed away.
Can you anodize 7075 or cast aluminium?
Yes, but the colour will not match 6061. 7075 contains copper and dyes toward darker grey; cast aluminium has high silicon and comes out blotchy. If a consistent colour matters, choose 6061 and choose it early — it cannot be corrected at the anodizing tank.
Will anodizing change my bore and thread dimensions?
It will. Anodizing grows about half the film outward and half inward, so a 20 µm film takes roughly 20 µm off a bore diameter and adds it to a shaft. Mask or re-tap threads, oversize precision bores, and put the finished dimension on the drawing rather than the pre-plate one.
Which finish should I choose for corrosion on steel?
Zinc plating for general outdoor exposure, nickel plating where wear accompanies the corrosion, and black oxide only for tooling or lightly protected indoor parts — black oxide alone is not a corrosion barrier. Yellow chromate over zinc is the common upgrade.
Can you apply two finishes to the same part?
Yes, and it is routine — bead blasting before anodizing, yellow chromate over zinc plating, or selective masking so a wear face gets hard anodizing while the rest of the part keeps its colour. Tell us the sequence you need and we will confirm the order.
Get a surface finishing quote for your machined parts
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