Polishing aluminum parts to a very high, near mirror finish in a mass finishing process is very challenging. The relative softness of aluminum makes the metal very susceptible to scratching and impact marks from the media and the vibratory or barrel tumbling process itself.
Depending on the original surface finish, a two or three step process is recommended. The first step with an abrasive Plastic or Synthetic Media will remove machine marks, scratches, etc. from the part. The second step using a polishing media (Ceramic or Steel) will create a smooth, uniform finish. An optional third step would involve a long cycle time dry tumbling of the part with Walnut Shell Grit or Corn Cob Grit to burnish to a high polish. Additives such as rouge can be added in this final step to improve the finish.
The part pictured was polished in a vibratory tumbler using Steel Media.