A pharmaceutical OEM in Frankfurt shipped 84 stainless enclosures for a cleanroom line in 2024. The drawing said "stainless steel, brushed finish." The buyer assumed No.4 brushed. The OEM quoted BA (bright anneal). The fabricator delivered 2B (mill finish). Three different surface finishes, three different costs, three different looks — and three weeks of email before anyone agreed on which one was actually specified. The buyer ended up with No.4 brushed on the doors and 2B on the side panels, both fully usable, neither matching what was originally drawn.
Stainless steel surface finish codes are a vocabulary problem disguised as a material spec. "Brushed," "satin," "matte," "polished" — every fabricator uses these words differently. The standard reference (ASTM A480, EN 10088-2, ISO 1302) defines a numerical scale — 2B, BA, No.3, No.4, No.6, No.7, No.8 — but most OEM drawings still call out the words, not the numbers, and the fabricator fills in whatever they have on the shelf. This guide is the cross-reference between the words and the numbers, with the Ra (roughness average) values, the cost premium per square meter, the applications where each finish makes sense, and the rules for matching finish to operating environment.
Stainless grade and stainless finish are two independent decisions. The grade (304 vs 316 vs 430 vs 904L) is covered in our stainless vs carbon steel guide; this guide is the finish half of that conversation. We will go through 6 standard finishes, the cost premium each one carries, the Ra values that procurement and QA teams can verify with a profilometer, and a 4-step finish-selection process that maps the drawing callout to the right mill-applied or fabricator-applied finish.
The 6 Standard Stainless Steel Sheet Finishes
ASTM A480 defines a numerical scale from No.1 (hot-rolled, annealed, pickled — the roughest) to No.8 (mirror-polished — the smoothest). For sheet-metal enclosures, six of these finishes cover 95% of OEM drawings. We list them in order of increasing smoothness and cost.
| Finish | Ra (µin) | Ra (µm) | Appearance | Process | Common use |
|---|---|---|---|---|---|
| 2B (mill) | 20–40 | 0.5–1.0 | Smooth, dull grey, slight reflectivity | Cold-rolled, annealed, pickled, skin-passed | Indoor enclosures, subplates, structural backsides |
| BA (bright anneal) | 10–20 | 0.25–0.5 | Mirror-like, highly reflective | Bright annealed in hydrogen atmosphere | Pharmaceutical, food-grade, decorative trim |
| No.3 | 40–80 | 1.0–2.0 | Coarse brushed, visible parallel lines | 120-grit abrasive belt | Heavy industrial, kitchen equipment, brewery |
| No.4 (brushed) | 20–50 | 0.5–1.3 | Fine brushed satin, parallel lines | 150–180-grit abrasive belt | Architectural panels, OEM enclosures, appliance |
| No.8 (mirror) | 2–8 | 0.05–0.2 | True mirror finish, reflective | Progressive polish to 1200+ grit, buffing | Decorative trim, food processing, reflective panels |
| Hairline | 10–25 | 0.25–0.6 | Long, continuous parallel lines | Specialty brush with long-stroke abrasive | Architectural interiors, elevator panels, signage |
The Ra numbers above are typical values measured per ISO 4287 / ASME Y14.36. Actual values from a mill coil can vary ±20% depending on the rolling pass, the belt grit, and the operator. When a drawing specifies "Ra ≤ 0.8 µm," request the mill cert and the in-house profilometer reading — never assume the mill 2B meets the spec without measurement.
Mill Finish vs Fabricator-Applied Finish
2B and BA are mill finishes — they come on the coil from the mill and the fabricator does not add cost to produce them. No.3, No.4, No.8, and hairline are fabricator-applied finishes — the fabricator runs the sheet through an abrasive belt line or a polishing line to create the texture. This is where the cost stacks up:
- 2B — $0 premium (mill default for cold-rolled stainless).
- BA — $0 premium (mill default for bright anneal; typically 8–12% above 2B because the mill annealing step uses hydrogen atmosphere).
- No.3 — $0.80–$1.50 / sq ft ($8–$16 / m²) for the abrasive belt pass; plus masking if only one side.
- No.4 — $1.00–$2.00 / sq ft ($11–$22 / m²) for the 150–180-grit belt pass; plus masking.
- No.8 (mirror) — $4.00–$8.00 / sq ft ($43–$86 / m²) for the progressive polish; plus masking; plus yield loss (mirrored sheets scratch easily in handling, so expect 5–8% scrap).
- Hairline — $1.50–$3.00 / sq ft ($16–$32 / m²) for the long-stroke abrasive; plus masking.
The masking note is critical. If only one side of the panel needs the brushed finish (the visible side of an enclosure door, for example), the back side must be masked during the abrasive pass. Masking adds 15–25% to the per-square-foot cost and is the most common line item left off a quote. Specify "No.4 brushed, front side only, back side mill finish 2B" and you will get the cost right.
How Finish Affects Corrosion Performance
Smoother is more corrosion-resistant. The mechanism: rougher surfaces trap contaminants, moisture, and chloride ions in the micro-valleys of the surface profile. Those trapped species break down the passive chromium oxide layer and start pitting corrosion. A No.4 brushed surface with Ra 0.8 µm has approximately 4× the surface area of a No.8 mirror with Ra 0.1 µm, and that extra area is where corrosion starts.
The practical implication for outdoor enclosures: 316 stainless with a No.4 brushed finish survives 1,500 hours of ASTM B117 salt spray. 316 stainless with a 2B mill finish survives 800 hours on the same test. 304 stainless with a 2B mill finish fails at 200 hours. The combination of grade (304 vs 316) and finish (2B vs No.4 vs No.8) is what determines the actual outdoor lifetime. For the full grade comparison, see our stainless vs carbon steel guide; for the broader coastal design rules, see our coastal EV enclosure guide; for the salt-spray test method itself, see our corrosion resistance coatings guide.
One important caveat: a polished finish is only as good as the underlying grain structure. A No.8 mirror on 304 stainless will still pit in a chloride environment, just slower than a 2B on 304. The grade matters more than the finish. The rule: pick the grade first (316 for chloride environments, 304 for everything else), then pick the finish for the cosmetic and cleanability requirement.
How Finish Affects Cleanability and Hygiene
For pharmaceutical, food processing, and cleanroom enclosures, cleanability is the dominant finish requirement. The smoother the finish, the easier to wipe down, the less bacteria can colonize the surface valleys. The hierarchy from cleanest to least clean: No.8 mirror > BA > No.4 brushed > 2B > No.3.
FDA and USDA guidance for food-contact surfaces recommends a finish at Ra ≤ 0.8 µm (32 µin), which corresponds to No.4 brushed at the fine end or BA. EHEDG (European Hygienic Engineering & Design Group) specifies Ra ≤ 0.8 µm for cleanability, with no surface cracks or crevices. A 2B mill finish is too rough for direct food contact; a No.4 brushed on 316L is the standard for pharmaceutical and food processing enclosures.
The finish direction also matters for cleanability. No.4 brushed has directional grain — the parallel lines run in one direction. Bacteria and contaminants accumulate at the bottom of the grooves. Cleaning must run along the grain, not across it. For a vertically oriented enclosure door, the grain should run vertically so cleaning wipes run top-to-bottom along the grain. For a horizontal work surface, the grain should run toward the drain. This is a drawing callout that costs nothing to specify but saves hours of cleanability validation.
The 4-Step Finish Selection Process
When an OEM drawing says "stainless steel, brushed finish," the fabricator has to make a decision. The decision is easier if the procurement or mechanical engineer runs through this 4-step process before sending the drawing to quote:
- Define the operating environment. Indoor industrial, outdoor general, coastal / chloride, pharmaceutical / cleanroom, food contact, architectural / decorative. Each environment has a default finish range.
- Define the cosmetic requirement. Hidden behind a gasket, visible from one side, visible from all sides, primary brand surface. Each visibility level has a default finish.
- Pick the grade. 304 for general indoor / outdoor, 316 for chloride / coastal / pharmaceutical, 430 for indoor dry architectural. The grade is independent of the finish.
- Pick the finish. Match the operating environment and cosmetic requirement to the table below.
| Environment | Grade | Default finish | Direction | Alternative |
|---|---|---|---|---|
| Indoor industrial, hidden behind gasket | 304 | 2B mill | — | — |
| Indoor industrial, visible one side | 304 | No.4 brushed | Vertical | 2B (lower cost) |
| Outdoor general, visible | 304 | No.4 brushed | Vertical | 2B + powder coat over stainless (not recommended) |
| Coastal / chloride, visible | 316 | No.4 brushed | Vertical or horizontal toward drain | No.8 mirror (premium) |
| Pharmaceutical / cleanroom | 316L | No.4 brushed | Along wipe direction | BA or No.8 |
| Food contact / processing | 316L | No.4 brushed | Toward drain | No.8 mirror (premium) |
| Architectural / decorative | 304 | Hairline | Vertical | No.8 mirror / BA |
| Architectural / decorative (high-traffic) | 316 | No.8 mirror | — | Hairline |
This table is the default. Specific customer specs (3-A Sanitary Standards, EHEDG, ASME BPE) may override the cosmetic line, but the grade and direction guidance holds. For the related decision on whether to use stainless or carbon steel in the first place, see our stainless vs carbon steel guide; for the cost trade-off across all enclosure materials, see our custom sheet metal enclosure cost guide.
Common Drawing Callouts — and What They Actually Mean
Most OEM drawings use informal finish callouts. Here is how to translate the words into the standard numerical codes:
- "Mill finish" or "as-rolled" → 2B (cold-rolled stainless default).
- "Brushed" or "satin" → No.4 (fine brushed, 150–180 grit). If the drawing does not specify grit, assume No.4.
- "Polished" → ambiguous: could mean No.4, No.6, No.7, or No.8 depending on the customer. Ask for the grit or the Ra value.
- "Mirror" → No.8 (true mirror). If the customer means "shiny," they probably want BA, not No.8 — confirm before quoting.
- "Hairline" or "linen" → hairline finish (long-stroke brushed). Specify direction (vertical, horizontal, along the long axis).
- "Bead blasted" or "matte" → glass-bead or aluminum-oxide bead blast, typically Ra 1.5–3.0 µm. Different from a brushed finish. Specify bead size and pressure.
- "Passivated" → not a finish; a chemical treatment (ASTM A967, ASTM A380) that removes free iron and enhances the chromium oxide layer. Specify separately from the finish.
The cleanest drawing callout for a stainless enclosure is something like: "316L stainless steel, No.4 brushed finish, vertical grain, front side only, back side 2B mill, passivated per ASTM A967." That callout leaves no ambiguity for the fabricator and no room for the mill to substitute.
Frequently Asked Questions
What is the difference between 2B and BA stainless steel?
2B is the standard cold-rolled, annealed, pickled, and skin-passed mill finish — smooth but dull grey. BA (bright anneal) is annealed in a hydrogen atmosphere at high temperature, which produces a mirror-like reflective surface. 2B has Ra 0.5–1.0 µm; BA has Ra 0.25–0.5 µm. Cost: BA carries a 8–12% mill premium over 2B. Application: 2B for hidden or industrial surfaces; BA for decorative, pharmaceutical, or food-contact surfaces.
What is the standard brushed finish for stainless steel enclosures?
No.4 brushed (150–180 grit abrasive belt) is the standard finish for OEM stainless enclosures. It has a fine satin appearance with parallel lines, Ra 0.5–1.3 µm, and costs $1.00–$2.00 per square foot above the mill 2B price. The grain direction should be specified on the drawing — vertical for doors, horizontal toward drain for work surfaces. No.3 (120 grit, coarser) is used for heavy industrial or kitchen equipment but is rarely the right choice for OEM enclosures.
Does a polished finish improve corrosion resistance?
Yes, but only marginally compared to grade selection. A No.8 mirror finish (Ra 0.05–0.2 µm) on 316 stainless survives roughly 1.5–2× longer in ASTM B117 salt spray than a 2B mill finish (Ra 0.5–1.0 µm) on the same 316 grade. But upgrading from 304 to 316 stainless improves salt-spray survival by roughly 4–6× regardless of finish. The rule: pick the grade first (316 for chloride environments), then pick the finish for cosmetic and cleanability requirements, not for corrosion performance.
Can I powder coat over stainless steel?
Yes, but it is rarely the right call. Powder coating over stainless eliminates the corrosion advantage of the stainless grade (the powder film fails in 5–8 years in chloride environments, exposing the stainless underneath to galvanic attack from the coating breakdown products). The one case where powder over stainless makes sense: cosmetic color matching on architectural panels where the stainless is structural and the powder is decorative. In every other case, specify the stainless finish directly (No.4, No.8, hairline) and skip the powder. For the broader powder coating decision, see our powder coating vs liquid painting guide.
Send Us Your Finish Specification
If you have a stainless enclosure drawing with a finish callout that the fabricator has been interpreting differently from what you intended, send the drawing and a one-paragraph description of the operating environment to gongtianbao@xhlmarketing.com or via the contact form. We will return the standardized finish callout (ASTM A480 numerical code, Ra value, grain direction, masking note), the cost premium per square meter for each side, and a fixed-price finish line item within 24 hours. Every stainless finish we quote comes out of a 12-year quote log that tracks which finishes survive which environments — we share that log with every quote review.