How Stainless Steel Surface Finishes Are Classified
Surface finish is one of the most under-specified items on a stainless steel order, and it is also one of the most visible. Two sheets of the same grade, 304 or 316L, can look completely different and behave differently in service simply because they left the mill with a different finish. The recognised finish designations come from ASTM A480 for flat-rolled products and from EN 10088-2 in Europe, with GB/T 3280 covering the same ground for Chinese deliveries.
Mill finishes such as No. 1, 2D, 2B and BA are produced by rolling and annealing. The polished finishes, No. 3, No. 4, No. 6, No. 7 and No. 8, are produced afterwards by abrasive grinding and buffing. The numbering is not a scale of quality; it is a scale of reflectivity and surface texture, and each step is chosen for the application rather than because it is higher up the list.
No. 3 Finish
No. 3 is a coarse ground finish, split into the 3A and 3B variants. 3A is a uniformly ground surface produced with an abrasive grit of roughly 80 to 100, giving a short, even scratch pattern. 3B is a wool-type finish with straight, parallel lines, usually produced by a single pass over a 2A or 2B base sheet with a belt in the 180 to 200 grit range. No. 3 is used where a strong, obviously machined appearance is wanted: industrial kitchens, lift interiors, heavy-duty work surfaces and equipment cladding.
No. 4 Finish
No. 4 is the most widely used polished finish in architectural and food-industry work. It has a unidirectional grain, a soft satin appearance and low reflectivity, which makes it forgiving of fingerprints and of minor handling marks. Production starts with a coarse abrasive and finishes at approximately 180 grit, and the scratch pattern runs in one direction along the sheet. Because the grain direction is visible, panels should be installed with a consistent orientation, and any repair work should follow the original direction.
No. 6 Finish
No. 6 is a further refinement of No. 4. The ground surface is treated with Tampico-type polishing brushes working in an abrasive and oil medium, which softens the grain and reduces its depth without producing a full mirror. It is a muted, dull satin finish. The No. 6 designation is listed in ASTM A480; it was not part of the older BS 1449 finish set, which has been withdrawn and replaced by the EN 10088-2 system. Where a No. 6 appearance is required, the practical route is to specify the roughness range rather than rely on the number alone.
No. 7 and No. 8 Finishes
No. 7 is a high-lustre buffed finish. The surface is first ground, then polished with a cloth or fibre wheel and a suitable polishing paste, giving a bright, near-mirror appearance that still retains faint polishing lines. It is often used for architectural trim and decorative panels where full mirror reflectivity is not necessary.
No. 8 is the mirror finish. The steel is polished progressively with fine abrasives and finished with a very fine polishing paste, producing a surface that reflects a clear image with essentially no visible grain. This is the finish used for decorative cladding, signage, column covers and interior features. It is also the least forgiving: in high-traffic or frequently touched areas it shows fingerprints clearly, and the surface scratches that would be invisible on a No. 4 grain stand out on a mirror. Cleaning is straightforward but has to be routine.
Roughness, Selection and Care
| Finish | Typical appearance | Indicative roughness | Common use |
|---|---|---|---|
| 2B | smooth, matte grey | 0.1 - 0.5 micrometre | general fabrication, kitchen equipment |
| No. 3 | coarse, uniform ground | 1.0 - 3.0 micrometre | heavy service surfaces, cladding |
| No. 4 | unidirectional satin | 0.4 - 1.0 micrometre | architecture, food processing, lifts |
| No. 6 | dull satin, softened grain | 0.5 - 1.5 micrometre | interior trim, appliances |
| No. 7 | high lustre, faint lines | 0.05 - 0.2 micrometre | decorative panels, trim |
| No. 8 | mirror, clear reflection | below 0.05 micrometre | feature cladding, signage, interiors |
The roughness values above are indicative ranges for guidance during finish selection; the controlling figures are those agreed for the specific order. Two practical rules apply to all polished finishes. First, the finer the finish, the more visible any later scratch, so handling protection matters more, not less. Second, a smoother surface is easier to clean and holds less soil, which is why the polished range is common in hygiene-sensitive areas.
Protect polished sheets with a removable film during fabrication and transport.
Grind and polish in one consistent direction and keep the direction constant across adjacent panels.
Use dedicated abrasive belts and wheels that have not been used on carbon steel, to avoid iron contamination and later rust spotting.
Clean with neutral detergents and avoid chloride-containing cleaners on any finish.
Frequently Asked Questions
Q: What is the difference between a No. 4 and a No. 8 finish?
No. 4 is a satin finish with a visible unidirectional grain and low reflectivity, while No. 8 is a mirror finish with essentially no grain and a clear reflective image.
Q: Which polished finish is the most commonly used?
No. 4 is the most widely specified polished finish, because its satin grain hides handling marks and fingerprints well in architectural and food-industry service.
Q: Does a mirror finish mean better corrosion resistance?
Not directly. A smoother surface retains less soil and drains better, which can help in cleanliness-critical service, but the corrosion resistance itself comes from the stainless grade.
Q: Why does a No. 6 finish sometimes differ between suppliers?
Because the designation describes a process rather than a single measured texture. Agreeing a roughness range removes that variability.
Q: Can a polished surface be repaired on site?
Local repair is practical for No. 3 and No. 4 using matched abrasives along the original grain. Mirror finishes are difficult to blend on site and are usually repaired by replacing the panel.
Q: How should polished stainless steel be cleaned?
Warm water with a mild neutral detergent, followed by a rinse and a soft cloth, is sufficient for routine cleaning in most indoor and outdoor applications.
