Machined aluminum parts end up with lines, swirls and etching on the surface of the aluminum. To prevent these surface imperfections from translating through a coating or anodizing, the surface needs to be made uniform. If a polished surface is ultimately required, abrasive blasting the surface with Glass Bead will produce a smooth finish.
The machine aluminum part (Before picture) was blasted with Fine (#13) Glass Bead to remove the machine lines and produce a highly uniform, matte surface finish (After picture). This parts was ultimately clear coated to produce a clean, bright finish. Different size of Glass Bead will each produce a unique surface finish on aluminum. As the Bead size increases, the surface finish will become more a peened finish and also brighter.