In order to obtain a good decorative finish after anodizing, the surface of the metal must be given a preliminary polishing treatment. In view of the softness and the tendency of aluminum to drag: its polishing requires special condition and the use of special polishing pastes. The peripheral speed of the grinding wheels is 30-50 ft./sec and that of the polishing wheels 50-85 ft./sec. For grinding, felt or cotton wheels with glued-on emery powder are used. The surface being ground is lubricated with a special lubricant. Polishing is done with pastes, the typical composition may be 71 percent chromic oxide, 23 percent stearine and 8 percent oleic acid.
Small parts are polished in bulk by tumbling in wooden barrels with steel balls. The amount of work treated is arranged to occupy 20-25 percent of the volume of the barrel, and the steel balls 75-85 percent. The polishing is done in present of soap emulsion containing 30 gram and 3 gram benzine per liter of water. The polishing is continued for 8-10 hours.