There are two commonly used preps for painting aluminum, at least in aviation. And sometimes they are used together.
Alodine wash. Usually considered a pre-treatment, and most times the yellow-orange color is used so that you can see that it has been done. Also available in clear (as mentioned above).
Zinc Chromate primer. This is usually a yellow or green primer that has a high content of zinc solids. This can go on top of Alodine or can be used alone. The idea is that the zinc element is more "sacrificial" than aluminum, and so will act as a protective layer. Like having a zinc block bolted to your boat engine.
Both products contain chromium and both "contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm."