No. 4 Finish Stainless Steel is the workhorse of the light fabrication industry. The easiest of the finishes to maintain, No. 4 finish is used for work surfaces, handrails, and where appearence is important.
A 'No. 4' surface is produced by cutting the surface with abrasive belts to remove a very small amount of metal without affecting its thickness.
For architects and designers, No. 4 finish gives low gloss and best apparent flatness of panels. For fabricators, the No. 4 finish is directional, allowing easy matching of surfaces and refinishing of welds. For end users, the surface can be repaired to remove any service damage.
No. 4 finish is duller than the other common finishes, 2B and BA and is generally used where lower relectivity or glass is required and where welds and other fabrication marks are to be refinished to match the original surface. This is not possible with 2B and difficult with BA.
Abrasive belts have very fine grains of refactory material such as silica, alumina, and zirconia embedded in an adhesive layer on a flexible cloth or paper backing. The belts are wider than the stainless steel which is usually worked on as coil or sometimes in individual sheets. The steel is run slowly under rolls, on which the abrasive belts run.
The polishing machines at stainless steel mills lubricate the cutting action by flooding the strip with cold. This helps to keep it cool and gives a finer more uniform surface.
The variability of the process means not every No. 4 fnish looks the same, even from the same source. Different maufacturers use belts with different combinations of grit sizes and the finish can vary through the life of a set of belts.
Where it is important that the appearence of material matches on a job, it should all be taken from the same pack of sheets, used sequentially and in the same orientation. A reasonable match in appearance can be achieved more readily with No. 4 finish than with 2B or BA mill finishes.
