As previously suggested, two (2) coats of zinc chromate primer followed by whatever type of paint you want would be the traditional do it yourself proceedure for aluminum... however, while in Harbor Freight the other day, I noticed that they sell powder coat paint in several colors. That got me to thinking....if you know someone that can do powder coating, I would highly suggest that approach since, with the possible exception of epoxy based paints, after it's baked powder coating leaves a better, harder, more durable
coat than any traditional paint.