The process for painting aluminum is a total pain in the behind. If you do it right the paint will last a long time. However if you don't the paint will peel off in no time at all. ALuminum has a natural oxide coating on it's surface that protects it from corrosion. That's why you see so many aluminum boats that aren't painted, and why aluminum fuel tanks aren't painted. However, to paint it you have to remove the oxide which requires essentially etching the surface, cleaning, coating with a base coat and then painting. Here's something on the subject because I have been asked this question thousands of times about aluminum fuel tanks. New Boatbuilders Home Page - Hot Topics for boatbuilders
There are some good links on that page. After the links it's all about fuel tanks so unless you're interested you can skip it. Frankly I would follow the advice above and have it anodized. Anodizing will last almost forever.