Acid etch it. Some folks have had good results with various homebrew mixes, but my experience is only with the commercially available stuff - Zep-a-lume and Alumabrite.
It will leave a uniform bare aluminum color, and as long as there isn't anything amiss (stray electrical current, trapped salt or fresh water, etc.), the only oxidation that will occur will be the normal protective layer of aluminum oxide, not the white powdery substance that is indicative of a problem.