Surface finish is the line on the drawing buyers cross out the most often — and the one that drives the biggest cost differences between quotes. Here's a plain-English rundown of anodize, plating, powder coat, and the finishes you probably should have specified instead.

Anodize (Type II and Type III)

Anodizing is an electrochemical process that grows a hard oxide layer on aluminum. Type II (sulfuric acid anodize) gives you 0.0002–0.001" of coating in clear or dyed colors. Use it for most cosmetic aluminum parts. Type III (hardcoat) gives you 0.001–0.004" of much harder coating, typically gray or black. Use it where wear matters.

Anodize grows into the aluminum and out of it roughly 50/50, so a 0.002" hardcoat will close a hole by 0.002" in diameter. Account for it on tight-tolerance fits.

Chem-film (Alodine, MIL-DTL-5541)

A thin chromate conversion coating used on aluminum aerospace parts where electrical conductivity matters and anodize is too thick. RoHS-friendly Type II is hexavalent-chromium-free. Cheap, fast, mil-spec. Doesn't add color, so cosmetic parts still need anodize on top.

Zinc plating

Cheap corrosion protection for steel. Comes in clear, yellow, and black. Adds about 0.0003" per side. Common spec: ASTM B633. Don't use on parts where galling matters.

Nickel plating (electroless and electrolytic)

Electroless nickel (ENP) plates uniformly into blind holes and threads — the only plating that does. Common in oil & gas, food contact, and dies. Adds 0.0005–0.002" per side and is hard out of the bath, harder still after bake. Spec: AMS-C-26074 or ASTM B733.

Black oxide

Conversion coating for steel that turns the surface jet black with almost no dimensional change (0.000010"). Cheap, fast, gives some corrosion resistance when oiled. Common on tool steel, fasteners, and firearms components.

Powder coat

Electrostatically applied polymer baked on at 350–400°F. Very durable, huge color palette, hides surface imperfections. Adds 0.003–0.008" per side, which is a lot — mask threads and bores. Use for enclosures, brackets, and outdoor steel weldments.

Wet paint

Thinner than powder coat (0.001–0.003" per side), can hit cosmetic finishes powder coat can't (gloss, metallic, multi-coat). Slower, more expensive. Use for visible products and color-matched assemblies.

Passivation

Not a coating — a chemical bath that strips free iron from stainless steel and re-establishes the chrome oxide layer. Required on stainless aerospace and food parts. Spec: ASTM A967 or AMS 2700.

Bead blast and tumble finishes

Mechanical finishes that hide tool marks. Bead blast gives a uniform satin texture; vibratory tumbling deburrs and softens edges. Often combined with anodize or paint. Spec the bead size or vibration time if it matters cosmetically.

What to put on the drawing

Spec the standard, the type/class, the color, and any dimensional accommodations already made. Example: "Hardcoat anodize per MIL-A-8625 Type III Class 2, black, 0.002" nominal. Hole tolerances pre-anodize."

We do anodize, chem-film, zinc plating, black oxide, passivation, bead blast, and powder coat through trusted local partners — most with 3–5 day turnaround. Send your drawing and we'll quote the finish along with the part: request a quote.