The key to painting aluminum siding is all in the prep work. If you don't carefully clean the siding to get off all of the dirt, powder, and mildew off the siding before you paint it, the paint will not stick well and it will be peeling in just a couple of years.

To clean your aluminum siding use a pressure washer with one cup of bleach, one cup of TSP (a cleaner available at hardware stores) and one gallon of water. Pressure wash the siding and then switch over to clean water and rinse the washed areas.

Let the aluminum siding dry completely before paining. When it comes time to paint apply one coat of an exterior acrylic primer and follow that up with two coats of exterior acrylic top coat.

The cleaning is a lot of work. But it'll save you a lot of trouble in the long run.