No.4 (often written No. 4 or "brushed satin") is the most widely specified stainless steel finish in the world. It shows a short, uniform directional grain, is semi-reflective rather than mirror-like, and hides fingerprints and minor handling marks far better than a bright annealed (BA) surface.
How the finish is produced
A No.4 finish is not a single operation. It is the result of an abrasive sequence — each belt removes the scratch pattern left by the previous one, and each step is finer than the last:
- Surface preparation. Mill scale, weld discolouration and deep scratches must be removed first. On coil or sheet this is usually a wide-belt grinding station with a coarser grit.
- Intermediate grits. Two or three coated abrasive belts, each finer than the previous, build the uniform grain.
- Final grit. The last belt defines the visible texture. Change this and you change the finish — which is why the final grit number should be agreed with your customer before the line is built.
- Cleaning and drying. Residual abrasive and swarf must be removed, or the surface will rust in storage.
Which grit numbers you use depends on the base material, the incoming surface condition and the specification you are working to. There is no single correct set — this is why the belt sequence is normally the first thing we agree with a customer.
What usually goes wrong
- Inconsistent grain direction. Caused by unstable strip tension or worn contact rolls. It is visible as "tiger stripes" once panels are installed next to each other.
- Belt loading. When the belt clogs with metal fines it stops cutting and starts burnishing, leaving a patchy, over-polished look.
- Thickness variation across the width. Usually a levelling or crown problem upstream, not a polishing problem.
- Poor cleaning. The most expensive defect, because it only shows up weeks later in the customer's warehouse.
No.4 compared with hairline and SB
All three are "brushed" finishes, which is why they are so often confused:
- No.4 — short, broken grain. The most forgiving and the most common.
- Hairline (HL) — a long, continuous grain. More directional, more reflective, and far less tolerant of speed variation.
- Vibration / SB — non-directional. Hides fingerprints better than either, because it has no dominant light-reflecting direction.
Which machine produces it
No.4 is produced on either a sheet-to-sheet or coil-to-coil platform, depending on your throughput and material handling. The choice is driven by coil weight, required line speed and floor space — not by the finish itself. Both are capable of a true No.4.