You can start out with white vinegar in warm water. If that doesn't do the job, Mother's liquid metal polish and a headlight buffing wheel will lightly polish aluminum. If these fail, Nevr-Dull can help.

If you need to polish the aluminum to a high gloss, there are a few notable aircraft polishes that are commonly used; Cyclo Free Flite, Met-All and Rolite. With time and a randon orbit polishing system, the aluminum can be shined to a mirror finish. Other polishing systems include slow speed buffers, foam pads and various compounds.

If you have a Bison trailer, the finish is a clear coat over a stainless skin. If the coating is damaged by acid washing, scratches or abrasion, the affected stainless will tarnish and cannot be spot polished. The entire panel must be treated.   

There are many marine finishes that can be applied over the bare aluminum. They will slow the oxidation process, and protect the metal from stains. They also make the next clean up much easier, as more of the product is used.