Like most types of metal surfaces, sheet aluminum is hard, slick and nonporous. This makes it highly resistant to paint adhesion. Sheet aluminum must be pre-treated through abrasion to help encourage it to accept paint. Unfortunately, you can't abrade aluminum by sanding it because the surface is too hard. If you want to paint sheet aluminum, you'll need to use a special kind of primer to rough up the surface or you will end up with a finish that will flake and peel rather quickly.
Things You'll Need
Acrylic latex paint
Scrub any greasy or oily areas using a degreaser and a brush. Wash the entire sheet aluminum surface using a pressure washer. Allow the sheet aluminum to dry completely.
Apply a coat of etching primer using a four-inch paintbrush. Allow the surface to dry and cure for four full hours.
Wash the brush using warm water.
Apply acrylic latex paint in the same manner as you did the primer.
Do not use a bonding primer en lieu of the etching primer or the finish will flake and peel over time.