Guides / Materials

Choosing a Finish for a Billet Automotive Part

October 8, 2026

A finish is not a coat of paint added at the end. On a machined part it is a dimensional change, a cost line and, on some features, a decision that has to be made before the first cut. Here is what each common finish actually does to a billet automotive part.

Finish changes the part, not just the look

Anodize is not a film sitting on the aluminum. The process converts a layer of the surface itself into aluminum oxide, and that layer grows both into the part and out of it in roughly equal measure. A bore gets tighter, a shaft gets fatter, a thread gets shallower. On a feature with a running fit or a press fit, that growth has to be allowed for in the machined dimension, or the anodizer has to mask it off.

Masking is not free. Every bore, thread or datum face that has to stay bare aluminum is a feature someone has to plug, tape or cap by hand before the part goes in the tank, and that labor shows up as its own line on the finishing invoice. A drawing with four masked features costs more to finish than one with a single clear callout that says “finish all over.”

The common finishes

Typical figures for planning a drawing, not guarantees for a specific finisher or alloy.

Finish	Typical buildup	Best for	Watch out for
Clear anodize, type II	0.0002 to 0.0003 in, about half in and half out	Everyday aluminum brackets, covers, spacers	Small enough to ignore on loose fits, not on a bore or thread
Color anodize, type II	Same as clear, dye added after sealing	Parts where color is part of the spec	Dye lots vary, run parts that must match in one batch
Hard anodize, type III	0.001 to 0.002 in total, roughly half in and half out	Wear surfaces, sliding fits, abrasion resistance	Buildup is large enough to close a running fit if not allowed for
Powder coat	0.002 to 0.004 in, sits on the surface	Covers and brackets, not fitted features	Too thick for a bore, a thread or a close mating face
Zinc or nickel plating, steel parts	0.0002 to 0.0005 in	Corrosion resistance on steel brackets and fasteners	Plating shops have minimum batch sizes, ask before quoting one part
Bead blast or brushed finish	No meaningful dimensional change	Surface preparation before anodize, or a cosmetic finish on its own	Not a corrosion or wear finish by itself

Which finish for which part

Engine bay brackets. Clear or color anodize is the normal answer. It handles heat well within its service range and it does not add meaningful thickness to a bracket with no close fits.

Parts under the car in a salted climate. Hard anodize on aluminum, or zinc or nickel plating on steel, outlasts a clear anodize finish against road salt and standing moisture. Bare aluminum in this location corrodes faster than most people expect.

Parts that clamp against a painted surface. A masked or bare contact face keeps torque consistent. Anodize is harder and slightly more brittle than bare aluminum, and clamping through it can lead to the joint loosening as the finish burnishes down under load.

Parts that carry an electrical ground. Anodize is an electrical insulator. A bracket that has to complete a ground path needs that contact area masked off, or the finish left off that surface entirely, otherwise the connection is unreliable from the day the part is installed.

What to write on the drawing

• The finish and the type, for example “hard anodize, type III” rather than just “anodize”
• A callout for thickness or buildup range if the feature is a close fit
• Which surfaces, if any, are masked and left bare
• Whether the dimensions on the drawing are the finished size or the pre-finish machined size

That last line prevents most of the rework on a first order. A shop that machines to a drawing dimension assuming it is the finished size, when it was actually meant as the pre-finish size, delivers a part that is out of spec once it comes back from the anodizer.

Cost and lead time

Finishing is usually done off site at a separate shop, so it adds shipping time on both ends as well as the process time itself. A few extra days is typical, and a rush surcharge is common if the finisher has to break a normal batch schedule for one order.

On a prototype, it is often worth skipping the finish and running the part bare aluminum or bead blasted to check fit and function first. Add the finish once the design is confirmed for the production run. A finish added twice, once on a prototype that gets revised and again on the final part, costs more than waiting.

What to tell a shop

Give the finish, the type and whether the drawing dimensions are before or after that finish is applied. If a feature has to stay bare, mark it. A shop that machines automotive parts regularly will flag a bore or thread that looks too tight once the finish is accounted for, and catching that before the first part runs is cheaper than catching it after.

Send the part details and we will match shops that finish parts routinely rather than send them out blind. For the alloy decision that usually comes before the finish decision, read 6061 or 7075 for automotive brackets, or start with the buyer’s guide.

Frequently asked questions

Keep reading

• 4130 or 4140 Chromoly: Which Steel for a Machined Car Part
• Picking Between 6061 and 7075 for a Loaded Part