Start with function, not looks
I've lost count of parts where the finish spec was chosen because it looked nice on a render. The question that should come first is always: what is this surface doing? A sensor housing that lives outdoors needs corrosion protection. A wear surface needs hardness. A connector needs conductivity. A machine cover needs cosmetics. Pick the finish from the requirement, and the looks follow — most finishes look fine when done right.
Machined finishes: what the Ra number actually means
A plain machined finish is a real spec, not a placeholder. Ra 3.2 µm is the default 'no comment' finish — visible tool marks, fine for most structural parts. Ra 1.6 µm looks uniform to the eye and is the sensible default for visible surfaces. Ra 0.8 µm is where parts start to feel smooth and seal better; it's also where the cycle time starts climbing because the machine has to take fine finishing passes. Below Ra 0.4 µm you're in grinding or polishing territory — only spec it on functional surfaces like seal journals, not on the whole part.
Anodizing: hard, clear, or black
Anodizing is the default answer for aluminum, and it's usually the right one. The coating is grown from the aluminum itself, so it never peels, and it's electrically insulating. Hard anodizing (Type III) gives 25-50 µm of extremely hard coating — genuine wear protection on aluminum that sees sliding contact. Clear or black anodizing (Type II) is 8-25 µm and is about corrosion and cosmetics, not wear. The trap: anodizing adds thickness, so anodize after final machining and tell the shop the coating thickness so they can adjust the pre-coat dimensions. And remember — threads should always be masked or chased after anodizing, or the fit disappears.
Plating: zinc, nickel, and the caveats
For steel parts, zinc plating is the low-cost corrosion standard — a few microns, cheap, and it takes a passivate for extra salt-spray hours. Electroless nickel is the serious option: uniform thickness on complex geometry, hard, and it doubles as a wear surface. Two things bite buyers here. First, plating adds measurable thickness (typically 5-25 µm) that changes fits — plated threads and bores need pre-plate machining allowances. Second, some plating processes can introduce hydrogen embrittlement on hardened steel; if the part is a high-strength fastener or spring, say so in the RFQ so the shop bakes it properly.
Paint and powder coat
Powder coating is the workhorse for frames, covers, and enclosures: 60-120 µm, tough, good color range. Paint is thinner and more precise for cosmetic finishes. The rules are simple: mask or plug every threaded hole and precision bore before coating, and don't powder coat anything that needs to hold a close fit — the coating isn't uniform on edges and corners. If a machined face must stay bare for a seal or a datum, it needs masking, which is a manual step that adds cost and lead time.
The cost ladder
Roughly, from cheapest to most expensive: plain machined < black oxide (steel) < zinc plating < Type II anodize < powder coat < Type III hard anodize < electroless nickel < specialized coatings (PVD, DLC, chrome). The spread is big — the top of that list can be 10-15x the bottom per part. If the drawing says 'hard anodize' on a decorative part, question it. If it says 'PVD' on a cover, definitely question it. Match the finish to the function and the budget takes care of itself.
Frequently Asked Questions
What is the cheapest way to protect aluminum parts?
Type II clear or black anodizing, typically 8-25 µm. It's inexpensive, gives good corrosion resistance, and can't peel because it's grown from the base material. Below that, a plain machined finish on 6061 is fine indoors.
Does anodizing change part dimensions?
Yes. The coating grows outward (and slightly inward), typically 8-25 µm for Type II, 25-50 µm for Type III. Machine the part to pre-coat size, tell the shop the coating thickness, and mask or chase threads after anodizing.
Zinc plating or electroless nickel for steel parts?
Zinc for corrosion protection at the lowest cost. Electroless nickel when you also need wear resistance, uniform thickness on complex geometry, or a harder surface. Nickel costs several times more.
When is powder coating the wrong choice?
When the part needs close fits or precise datums. Powder coat is 60-120 µm and doesn't build evenly on edges, so any sealing face, bore, or datum must be masked — and masking adds cost. For precision parts, anodizing or plating is usually cleaner.