Bare aluminium is similar to vinyl, fibreglass and plastic in that it requires abrasion before any paint will adhere to it. Unlike wood, which is porous, these other surfaces are unable to soak up the adhesives within paint, and they will eventually shed their painted finishes unless they are properly prepared. Unfortunately, unlike these other nonporous surfaces, aluminium is hard and durable, making friction-based abrasion techniques like sanding useless. If you wish to paint bare aluminium, you must abrade it first by applying a special type of primer capable of etching the metal to provide a tooth for the paint to adhere to.
Clean the aluminium, using a sponge and cleaning agent of your choice. Ensure that the cleanser contains a water-based degreaser. Rinse the bare aluminium well with a hose, or you may have adhesion difficulties. Wait for the aluminium to dry, or dry it yourself with clean rags.
Coat the bare aluminium with a galvanised metal etching primer, using a latex paintbrush. Wait four to six hours for the metal etching primer to cure.
Wash the paintbrush with plain tap water.
Coat the primed aluminium with an oil-based enamel or acrylic latex paint. Apply oil-based paints with an oil-based paintbrush. Use a latex paintbrush with acrylic latex paints.
Wash oil-based paintbrushes with mineral spirits. Use water to clean latex paintbrushes.
Use a spray galvanised metal etching primer and spray oil-based enamel or acrylic latex paint if you are painting a small piece of bare aluminium.
Paint will chip and flake from bare aluminium unless it is coated with a galvanised metal etching primer before application.