Aluminum is pretty tricky to paint properly. Paint doesn't like to stick to it. That's why aluminum boats don't get painted. The grey coat is likely anodizing. If I was going to clean mine up I think I'd try to polish it out. The downside there is it sounds like a major pain and shiny metal retains more heat than black painted metal. Let me know how it goes, I have a couple that could use refinishing.

__________________
89 300te 222,222
92 300e 190,000