If you are specifying a stainless steel surface finish and only remember one thing, make it this: the finish is not decoration — it is a measurable surface roughness (Ra) that decides how the part resists corrosion, hides fingerprints and cleans. A mirror-polished 304 will still pit in chloride service, and a dull 2B 316L will still resist it. The grade buys corrosion resistance; the finish buys appearance, cleanability and, to a smaller degree, resistance.
The designations — 2B, BA, No.4, HL (hairline), 8K — are mill shorthand that different suppliers apply slightly differently. The number you can actually hold someone to is the Ra value in micrometers, and it is worth writing into the order.
What the designations mean
| Finish | How it is made | Typical Ra | Look | Where it goes |
|---|---|---|---|---|
| 2B | cold rolled, annealed, lightly skin-passed | 0.1 – 0.5 µm | matte, uniform gray | general sheet, hidden faces, workhorse |
| BA | bright annealed in a controlled atmosphere | 0.05 – 0.1 µm | bright, reflective | cosmetic trim, appliance panels |
| No.4 | mechanically polished with 150–180 grit abrasive | 0.3 – 0.5 µm | brushed, directional | architectural, food equipment |
| HL / Hairline | continuous linear brushing | 0.2 – 0.4 µm | fine unidirectional lines | elevators, cladding, consumer goods |
| 8K / mirror | multi-stage buffing to a mirror | below 0.05 µm | mirror | decorative, signage, high-end trim |
The pattern behind the table: lower Ra means brighter and harder to keep clean-looking, but also harder to make consistently. Every extra polishing pass adds cost and adds reject risk, because each abrasion step is another chance for a scratch or a visible line where two sheets meet.
The standards that pin these down differ by system. The ASTM route defines finish and dimensional tolerance in ASTM A480/A480M; the EN route uses the surface designations of EN 10088-2, with cold-rolled tolerances from EN ISO 9445-2 and hot-rolled from EN 10051. When the drawing names one system, do not accept a certificate written against the other without checking the mapping.
What the finish actually controls
Corrosion resistance, but only a little. A smooth surface holds less moisture and fewer crevices than a rough one, so a finer finish resists staining marginally better. This is real but secondary — the grade, not the polish, is what stops pitting.
Cleanability. This is where Ra matters most. Food and pharmaceutical equipment is specified to a surface where bacteria have nothing to grip. A 2B sheet at Ra 0.4 µm will not pass a hygiene specification written for Ra 0.3 µm, no matter how it looks.
How it hides or shows fingerprints. A mirror finish shows every fingerprint; a No.4 brushed finish hides them. The "premium" finish is often the wrong one for a touched surface.
Grain direction. Brushed and hairline finishes have a directional grain. Every sheet in a run must be laid with the grain running the same way, or the panel reads as two different colours under light. This is the single most common finishing complaint on architectural jobs.
How to write the finish into the order
State four things: the grade, the finish designation, the Ra target, and the grain direction. "304, 2B, Ra ≤ 0.4 µm" is a complete call-out. "304, brushed" is not — it does not tell the mill what grit, what direction, or what roughness you will accept.
For a mirror or hairline finish that will be cut after finishing, say so. Cut edges on a mirror or hairline sheet are raw and will not match the polished face, so the part either needs the edges dressed or the finish applied after cutting. Leaving this to assumption is how a job ships with bright faces and dull edges.
What gets specified wrong
The most common mistake is ordering "No.4" and expecting the same finish from two different mills. No.4 is a grit range, not a single roughness, and 150-grit from one supplier looks different from 180-grit from another. If the appearance matters, write the Ra and a reference sample into the order instead of trusting the designation.
The second mistake is specifying a mirror finish for a part that gets touched. 8K shows every fingerprint and every micro-scratch; on a door panel or lift wall it looks worse in service than a hairline finish would, at several times the cost.
The third mistake is ignoring grain direction on a multi-sheet installation. Sheets laid with crossed grain read as mismatched panels. On an architectural job the fix is one sentence in the order: "grain to run vertically, all sheets."
The decision in one table
| You need | Choose | Write in the order |
|---|---|---|
| General sheet, hidden or over-clad | 2B | grade + 2B |
| Bright cosmetic, no touching | BA | grade + BA + Ra |
| Architectural, food, brushed look | No.4 | grade + No.4 + grit + grain direction |
| Elevator, cladding, consumer | HL hairline | grade + HL + grain direction |
| Decorative, signage, high-end | 8K mirror | grade + 8K + Ra ≤ 0.05 µm |
Bottom line
The finish is a specification, not a preference. Pick the grade for corrosion, then pick the finish for cleanability, appearance and cost — and write the Ra and grain direction into the order, not just the designation. When the appearance genuinely matters, ask for a reference sample and hold the mill to it.
For the stainless steel sheet and plate spec range and the color coated aluminum comparison, see the category pages. If you are quoting a finished part, send the finish designation, the Ra target and the grain direction — we will confirm which mill can hold it and at what cost.