Thoroughly clean the aluminum and let dry. Sand the metal surface with coarse-, then fine-grit sandpaper. Apply self-etching primer, let dry, then sand again. Apply paint (multiple coats, and sanding between coats, may be necessary). Apply enamel sealer.

Buy acrylic or latex paint. For best results, choose paint with a matte or satin finish. While glossy paint will work, it is not recommended because it will show off any imperfections in the metal. Because you already applied the primer, you can use any type of paint you want. It does not have to be labeled "for metal."