Mix 1/4 cup of non-phosphate powdered detergent into a large bucket of water. Wet a sponge with the mixture and wash it over the aluminum. Scrub the aluminum with a stiff brush to loosen any dirt and rust on the surface. Rinse the aluminum with a garden hose. Allow the aluminum to dry over a two-day period.
Wet a paintbrush or roller with an alkyd-based primer. Brush or roll the primer onto the aluminum, starting at the top and working down. Allow it to dry for two hours.
Apply a 100-percent acrylic latex paint to the aluminum. Brush or roll the paint onto the aluminum the same way as the primer. Allow the paint to dry for one to two hours, and then apply a second coat.