Tips for painting aluminum siding
The key to painting aluminum siding is to get it really clean and to use 100-percent acrylic latex paint. Check the label to be sure it's recommended for metal siding. Expect to pay the premium for good paint.
The best way to get this siding clean is to power wash it. Or you could wash it by hand with TSP or a similar product (available at home centers and paint stores) and rinse it thoroughly. As long as the siding is clean, you won't need to use a primer unless raw metal is exposed. Finally, since metal is such a good heat absorber, avoid painting siding while it's getting direct sunlight.