A satin finish, also called #4 finish, is a brushed finish which can often differ in appearance from batch to batch due to manufacturing variations. The satin appearance is characterised by a pattern of very fine parallel lines, producing a decorative appearance which is not too reflective.
The #4 brushed finish is used on stainless steel applications where appearance is important, but reflectiveness is unwanted. For example, kitchen benchtops, hand rails, architecture or automotives.
As a satin finish is created by creating fine scratches in the stainless steel, this does make it slightly less corrosion resistant than a mirror finish as the groves are susceptible to rust.
#3 is also a brushed finish, but has a much more grainy look, with thicker parallel scratch marks, as seen in the image to the right
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