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Surface roughness is never decided by machining alone. Every finishing step that follows — anodizing, plating, blasting, passivation, powder coating — either smooths the surface, roughens it, faithfully copies it, or buries it under a new layer. Understanding which coating and surface treatment processes do what to Ra is the difference between a part that passes inspection and one that comes back from the plater out of spec. This guide explains the mechanism behind each process family and shows how to specify roughness correctly before and after treatment.
Ra (arithmetic average roughness, per ISO 21920-2) is the number most drawings use to describe a surface. Before any coating is applied, the machining process sets the starting point:
This baseline matters because most thin coatings are conformal — they replicate the topography underneath. A rough substrate going in means a rough coated surface coming out.
Polishing physically removes the peaks of the machined profile and can take a ground surface down to a mirror finish of Ra 0.1–0.2 µm. It is the most direct way to lower Ra, and it is often the required preparation before decorative plating or anodizing, because thin coatings will telegraph every scratch that polishing would have removed.
Electropolishing works in reverse at the microscopic level: the electrochemical bath dissolves the micro-peaks faster than the valleys, removing roughly 5–25 µm of material and leaving a bright, smooth profile in the range of Ra 0.1–0.4 µm on stainless steel. Because it preferentially attacks high points, it lowers Ra while also improving corrosion resistance — which is why it is the standard finish for medical, food-contact, and semiconductor components.
Blasting deliberately roughens the surface to a uniform matte texture, typically Ra 1.6–6.3 µm depending on media type, mesh size, and pressure. Finer media and lower pressure produce a lower Ra; coarse grit produces a higher one. The added texture is not a defect — it hides tool marks and fingerprints, and the micro-anchor profile it creates measurably improves the adhesion of subsequent paint or powder coating.
Anodizing converts the aluminum surface into aluminum oxide, and the newly formed oxide phase is not perfectly uniform — an anodized surface is generally rougher than the substrate it grew from. Decorative Type II anodize at 8–25 µm thick adds only a slight increase in Ra, but Type III hardcoat at 25–75 µm can roughen the surface noticeably and grows about half of its thickness outward, adding roughly 0.025 mm per surface on a typical 2-mil coating. If a sealing or bearing face must stay smooth, polish it before anodizing and state the final Ra requirement clearly on the drawing.
Several common treatments add so little material, or deposit it so uniformly, that the machined Ra survives essentially unchanged:
Powder coating applies a 50–100 µm film cured at 175–200 °C. At that thickness the coating no longer follows the machined profile — it hides fine tool marks entirely, but replaces them with its own characteristic "orange peel" texture, and it will cover roughly 0.05–0.1 mm per surface, which rules it out for press fits and threaded features unless they are masked. Wet paint and e-coat behave the same way at thinner builds. The practical rule: when the coating is thick, specify the substrate smoothness for adhesion (a light blast is ideal), not for the final appearance.
| Process | Typical Build | Effect on Surface Roughness |
|---|---|---|
| Polishing / lapping | Removes material | Decreases Ra, down to 0.1–0.2 µm mirror finish |
| Electropolishing | Removes 5–25 µm | Decreases Ra, typically 0.1–0.4 µm on stainless steel |
| Bead blasting | None (texture only) | Increases Ra to 1.6–6.3 µm, uniform matte |
| Anodizing Type II / III | 8–25 µm / 25–75 µm | Slightly increases Ra; hardcoat more so |
| Passivation | None | No change in Ra |
| Black oxide / chromate | <3 µm | Negligible change |
| Zinc / electroless nickel plating | 5–25 µm | Follows substrate; thicker coats level micro-scratches |
| Powder coating | 50–100 µm | Hides machined texture; adds its own orange-peel finish |
Even when a coating transforms the surface, the roughness underneath continues to govern three things. First, adhesion: paint and powder coat bond best to a light anchor profile, while plating adheres best to a clean, smooth base. Second, appearance: thin conformal coatings telegraph every defect, so a decorative anodized part should be polished or fine-blasted before it ever enters the tank. Third, function: sealing faces, bearing seats, and fatigue-critical surfaces need their final Ra verified after treatment, not assumed from the machined value. The correct workflow is to specify the pre-treatment Ra on the drawing, state the finish with its standard and thickness, call out any masking, and note whether dimensions apply before or after coating.
Managing roughness across machining and finishing is far easier when both happen under one roof. ANOK Precision Manufacturing provides in-house coating and surface treatment alongside CNC machining — including anodizing, electroplating, powder coating, passivation, blackening, phosphating, nitriding, and PVD/CVD — with dimensional variance on aluminum treatments held under 5 µm. For surfaces that must stay smooth before and after coating, our precision surface grinding holds tolerances within ±0.002 mm and roughness down to Ra 0.4, with mirror polishing to Ra 0.2 available. As an ISO 9001:2015 certified factory, we control the entire chain from as-machined Ra to final coated finish, so the number on your drawing is the number on your part. Send us your drawings and finish specifications — our engineers will recommend the right process combination and confirm the achievable roughness before production begins.