“Stainless steel” doesn’t tell you what the surface looks or behaves like — that’s set by the finish. The same 304 sheet can arrive as a dull industrial surface, a brushed satin panel, or a flawless mirror, and the choice affects cleanability, corrosion resistance, appearance, and cost. Specify the wrong finish and you either pay for a mirror polish a hidden bracket never needed, or get a rough surface that can’t meet a hygiene requirement.
This guide explains the common stainless finishes — 2B, BA, No.4, and No.8 — what their Ra roughness values mean, and how to choose. The chart and framework at the end make specifying straightforward.
The quick answer 2B — smooth mill finish (Ra ~0.4–0.8 µm), the economical industrial default. BA — bright annealed, smoother and reflective, for a clean bright look. No.4 — brushed satin with a directional grain, hides fingerprints; the food-equipment and architectural workhorse. No.8 — mirror polish (Ra ≤0.2–0.4 µm), for decorative and high-hygiene uses. Electropolished — lowest Ra, best cleanability, for medical and pharma. Rule: finish where it matters. Smoother = cleaner and more corrosion-resistant, but each step adds cost.
What Ra Means
Before the finishes, one term: Ra (roughness average). It quantifies the microscopic irregularities on the surface, in micrometers (µm). The key relationship:
Lower Ra = smoother surface = easier to clean and more corrosion-resistant.
A smoother surface has fewer microscopic “valleys” where contaminants, bacteria, and chlorides can settle — which is why smoother finishes resist pitting corrosion better and sanitize more easily. This is the thread connecting all the finishes below.
The Common Finishes
No.1 — Hot-rolled (the starting point)
Hot-rolled, annealed, and descaled — a rough, dull surface. Rarely a final finish; it’s the starting point for higher grades, or used where appearance doesn’t matter and heat/structural strength does.
2B — Smooth mill finish (the default)
Cold-rolled, annealed, pickled, and given a light skin-pass. Ra typically 0.4–0.8 µm. This is the most widely used industrial finish — a smooth, slightly reflective surface that balances smoothness, corrosion resistance, and cost. The default for tanks, process equipment, and general fabrication, and a common starting point for food and hygienic work.
BA — Bright Annealed
Annealed in a protective atmosphere to prevent oxidation, producing a bright, smooth, reflective surface. Smoother and more reflective than 2B, but a mill finish — consistent and cost-effective, though it may carry a slight haze versus a polished mirror. Good for appliances and visible parts needing brightness without a full polish.
No.4 — Brushed / Satin (the workhorse)
Mechanically polished with abrasive belts to a uniform, directional brushed grain. The most popular functional finish. Its short, broken grit lines hide fingerprints and scratches well, making it ideal for high-traffic surfaces. It’s the standard for food equipment, kitchen surfaces, medical carts, handrails, and architectural panels — the best all-round balance of cost, corrosion resistance, cleanability, and appearance.
No.8 / 8K — Mirror
Multiple stages of fine polishing and buffing to a highly reflective, near-distortion-free mirror. Ra ≤0.2–0.4 µm. The smoothest mechanical finish — used for decorative architecture, and for high-hygiene systems (pharma, semiconductor, food-grade) where the low roughness aids cleanability. Resists corrosion well but shows smudges and scratches.
Electropolished
An electrochemical process that removes a thin surface layer, leaving the lowest Ra and best cleanability. Preferred for medical devices, pharmaceutical, and the most demanding hygienic systems — it also passivates the surface, maximizing corrosion resistance. (See our passivation guide.)
The Finish Chart
| Finish | Typical Ra (µm) | Look | Best for |
|---|---|---|---|
| No.1 | Rough | Dull hot-rolled | Structural, non-visible |
| 2B | 0.4–0.8 | Smooth, slightly reflective | Industrial, tanks, process |
| BA | ~0.1–0.5 | Bright, reflective | Appliances, bright visible parts |
| No.4 | ~0.4–1.0 | Brushed satin grain | Food equipment, medical carts, architecture |
| No.8 | ≤0.2–0.4 | Mirror | Decorative, pharma, high-hygiene |
| Electropolished | ≤0.1–0.4 | Bright, micro-smooth | Medical, pharma, critical hygiene |
Ra ranges are indicative — grit size, line speed, and process vary results. Always confirm with actual Ra measurement.
Finish, Hygiene, and Corrosion
For hygienic applications, the finish isn’t cosmetic — it’s functional:
- Food and beverage: the commonly accepted target is Ra ≤ 0.8 µm. 2B, No.4, BA, and electropolished all see use depending on contact and visibility.
- Pharmaceutical, dairy, bioprocessing: more demanding — Ra ≤ 0.5 µm or lower, often electropolished.
- Corrosion: smoother finishes (lower Ra) resist pitting and chloride attack better, because there are fewer crevices for contaminants to settle — relevant in marine, chemical, and hygienic settings.
Passivation or electropolishing after fabrication further improves both corrosion resistance and cleanliness — important because welding and forming can compromise the surface.
Cost: Finish Where It Matters
Each finishing step adds time, labor, and cost. Approximate increases over a 2B base:
| Finish | Cost increase over 2B |
|---|---|
| No.4 | +20–30% |
| No.6 | +50% |
| No.8 / PVD color | +200–300% |
The smart strategy is “finish where it matters” — apply high-end finishes only on visible or hygiene-critical surfaces, and keep hidden or structural surfaces at mill/2B levels. A mirror polish on the inside of a sealed cabinet is wasted money; a No.4 on the visible exterior of a medical cart is exactly right.
How to Specify (Avoid the Vague-Term Trap)
Vague terms like “polished” lead to inconsistent results between suppliers. Specify clearly:
- Finish name (2B, BA, No.4, HL, No.8) — and grain direction for brushed finishes
- Ra limit where hygiene matters (e.g. “max 0.6 µm”)
- Grade (304, 316L, etc.)
- Protective film (PE/PVC) for polished sheet
- Inspection (visual approval, Ra report, MTC)
A complete spec ensures the finish you get matches the finish you designed.
The Decision Framework
- Cost is the priority / non-visible → 2B (or No.1 for structural)
- Bright appearance, no full polish → BA
- Best all-round (visible + functional + hygienic) → No.4
- Decorative or high-hygiene, mirror needed → No.8
- Medical / pharma / critical cleanability → electropolished
- Food contact → Ra ≤ 0.8 µm (No.4, BA, or electropolished); pharma ≤ 0.5 µm
How DRAmetal Helps
DRAmetal reviews the required stainless finish, intended environment, visible surfaces and inspection needs against the submitted drawing and the available project-specific finishing route:
- 2B, BA, No.4 and No.8 material or finishing availability confirmed against the specified grade and supplier.
- “Finish where it matters” guidance — high-end finish on visible/critical surfaces, economical elsewhere
- Roughness acceptance defined by the approved Ra target, measurement method, sampling locations and reporting scope.
- Passivation or electropolishing evaluated against material compatibility, finishing-provider availability and the agreed verification requirements.
These decisions are relevant to medical cart structures and medical device enclosures, where the OEM must define and validate application-specific cleanability requirements. The medical equipment sheet metal parts page covers the RFQ boundary for drawing-controlled brackets, trays, plates and kits. For material selection, see our 304 vs 316 stainless guide and stainless passivation guide.
Not sure which finish your part needs? Send us your drawing and application — including hygiene requirements and visible surfaces. The initial response target is within 24 working hours; finish recommendations, DFM and quotation timing depend on the submitted scope.
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