Reference

Surface Finishes Compared

The common finishes for machined parts side by side by purpose, compatible materials, and whether they add thickness, so you can pick the right one and dig into the detail spec.

A finish protects a part, changes how it looks, or improves how it wears, and the right choice depends on the base metal and the job. Some finishes convert or clean the surface and add no thickness (anodizing, passivation, bead blast); others deposit a film or coating that grows the part and can matter on tight tolerances (plating, powder coat, hardcoat). This chart compares the common options so you can narrow it down, then follow the links to the detailed spec for the one you need.

Common finishes compared

Anodizing is for aluminum only; plating and passivation are mostly for steels and stainless; powder coat and bead blast work across most metals. Thickness figures are typical ranges; confirm on your part.

Finish	Purpose	Materials	Adds thickness?	Notes
As-machined (bare)	None; raw machined surface	Any	No	Bare metal, ~3.2 um (125 uin) Ra typical; will oxidize or corrode over time
Type II anodize	Corrosion protection and color	Aluminum	Yes, ~5-25 um	Integral oxide, dyeable; will not chip; MIL-A-8625
Type III hardcoat anodize	Wear and abrasion resistance	Aluminum	Yes, ~25-100 um	Hard, thick, gray to black; about half grows outward per side; MIL-A-8625
Powder coat	Durable colored film	Steel, aluminum, most metals	Yes, ~50-100 um (2-4 mil)	Wide color range; can chip; adhesion depends on pretreatment
Zinc plating	Sacrificial corrosion protection	Steel, iron	Yes, ~5-25 um	Usually sealed with a chromate/passivate; ASTM B633
Electroless nickel	Uniform corrosion and wear resistance	Steel, aluminum, copper alloys	Yes, ~5-75 um	Plates evenly on complex shapes and bores; ASTM B733
Passivation	Restore stainless corrosion resistance	Stainless steel	No	Chemical clean, not a coating; adds no measurable thickness; ASTM A967
Bead blast	Uniform matte cosmetic texture	Most metals	No (removes a trace)	Cosmetic; often used as a base texture before anodize or coating

Typical values and uses. Achievable thickness, color, and texture depend on alloy, geometry, and shop. Confirm on your part before release.

Which finishes change your dimensions

This is the first thing to check on a toleranced feature. Conversion and cleaning processes add essentially no thickness: anodizing converts the surface (Type II and III do grow the part, but roughly half the coating thickness grows outward per side, so it is predictable and often masked); passivation adds none; bead blast removes a trace. Deposited coatings add real material: powder coat is thick (roughly 2 to 4 mil / 50 to 100 um) and usually masked on mating faces and threads, while plating adds a few microns to tens of microns depending on the class. Call out masking on threaded holes, bores, and datum surfaces wherever the added film would push a feature out of tolerance.

Picking a finish by goal

Start from what the part needs. For corrosion protection on steel, use zinc or zinc-nickel plating, or powder coat for a colored barrier. For wear and hardness, use Type III hardcoat anodize on aluminum or electroless nickel / hard chrome on steel. For color and appearance, use Type II anodize on aluminum or powder coat on almost anything. For stainless that just needs its natural corrosion resistance restored after machining, use passivation. For a clean, uniform cosmetic look with no dimensional change, use bead blast. When in doubt, name the environment and the function and let the shop recommend a finish.

Values reflect MIL-A-8625, ASTM B633, ASTM B733, ASTM A967, and common shop practice, consistent with the detailed finish pages linked below. Confirm the exact finish, thickness, and masking a shop can hold on your material before release.

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Frequently Asked Questions

Which surface finishes add thickness to a part?

Plating (zinc, electroless nickel) and powder coat add real material, from a few microns up to about 100 microns. Anodizing grows the part too, roughly half the coating thickness per side. Passivation and bead blast add essentially no thickness. Mask threads and toleranced features on the coatings that build up.

What is the best finish for aluminum parts?

It depends on the goal. Type II anodize gives corrosion protection and color; Type III hardcoat anodize adds wear resistance; powder coat gives bold color across mixed alloys and castings but can chip. Anodizing is integral and will not peel, so it is the usual choice for a hard, thin finish on wrought aluminum.

What finish protects steel from rust?

Zinc or zinc-nickel plating gives sacrificial corrosion protection and is common on fasteners and hardware. Electroless nickel adds corrosion and wear resistance with very uniform coverage. Powder coat provides a thick colored barrier. The right pick depends on the service environment and whether you need color.

Do I always need a finish on a machined part?

No. Many parts ship as-machined (bare), especially for prototypes and non-corrosive service. But bare steel will rust and bare aluminum oxidizes, so add a finish when the part sees moisture, wear, handling, or needs a specific look. Stainless often just needs passivation to restore its corrosion resistance.

Tell us your material, the finish and color you need, and where the part will live, and set a budget. We source shops that do the finish and bring back a best-match quote.

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