Surface finish is where a part stops being a blank and starts being a product. It drives appearance, cleanability, corrosion resistance — and cost. This guide covers the finishes we deliver on sheet metal, their typical Ra (roughness) values, and how to specify the right one on your drawing so the quote and the result match your intent.
What Ra means (in one line)
Ra is average surface roughness in micrometres (µm). Lower Ra = smoother and more reflective. A mirror finish is well under 0.05µm; a matte blasted finish can be several µm. You rarely need to name an Ra number — naming the standard finish is enough — but it’s the language behind the look.
Mill finishes (as-supplied)
| Finish | Typical Ra | Appearance | Use |
|---|---|---|---|
| 2B (cold-rolled) | 0.1 – 0.5µm | Dull, smooth silver | Default stainless; base for coating or further finishing |
| BA (bright annealed) | 0.05 – 0.1µm | Bright, reflective | Hygienic and decorative parts |
Mechanical finishes
| Finish | Typical Ra | Appearance | Use |
|---|---|---|---|
| #4 brushed (150–180 grit) | 0.4 – 0.6µm | Uniform directional satin | Appliances, kitchens, architecture |
| Hairline (HL) | 0.2 – 0.4µm | Fine, long continuous grain | Elevator panels, façades |
| #8 mirror | ≤ 0.05µm | Mirror reflective | Decorative, signage, reflectors |
| Bead / sand blasted | 1 – 3µm | Uniform matte, non-directional | Even matte look, hides handling marks |
The most common request for stainless products is #4 brushed — it looks clean, hides fingerprints better than mirror, and is repeatable across panels.
Coatings
For steel and aluminum that need color or corrosion protection, powder coating is the workhorse:
| Spec | Typical value |
|---|---|
| Film thickness | 60 – 120µm (2.4 – 4.7 mil) |
| Color | RAL palette (200+ standard colors) |
| Cure | Electrostatic spray + oven cure ~180 – 200°C |
| Properties | Corrosion + UV resistant, tougher than wet paint |
Full breakdown and pricing: powder coating guide. We run coating in-house, so cut → bend → weld → coat is one job, not three suppliers.
Laser-cut edge finishes
The cut edge itself is a finish you specify by assist gas:
- Nitrogen (N₂) edge — clean, bright, oxide-free. Ready to weld, brush, or coat with no prep. Standard for stainless and aluminum.
- Oxygen (O₂) edge — used on thick steel; leaves a thin oxide layer that must be ground before painting.
See how this ties to thickness in the tolerance & kerf chart.
How to specify finish on your drawing
- Name the standard finish — “Stainless #4 brushed, grain along long axis” beats “make it shiny.”
- State the visible faces — finishing only the faces that show saves money.
- For coating, give the RAL number and gloss level (e.g. “RAL 9005 matte”).
- Call out grain direction on brushed parts that sit next to each other.
FAQ
What is the difference between #4 and mirror (#8) finish? #4 is a directional brushed satin (Ra ~0.4–0.6µm); #8 is a polished mirror (Ra ≤0.05µm). #4 hides fingerprints and is cheaper; #8 is decorative.
What powder coat thickness is standard? 60–120µm (about 2.4–4.7 mil), depending on indoor vs outdoor use.
Do I need to specify an Ra number? Usually no — naming the standard finish (2B, #4, mirror) is enough. Specify Ra only for sealing or hygienic surfaces.
Get the finish you want
Tell us the finish and visible faces with your part and we’ll quote it as one job. Request a quote or see pricing.
