Oil-Based Metal Primer
Outdoor aluminum should first be primed with an oil-based priming paint made for use on metal. Acrylic latex paint is often used for outdoor painting. However, when the ammonia in acrylic latex paint comes into contact with aluminum, it creates tiny bubbles of gas which become trapped between the aluminum and the paint and prevent it from adhering properly. Therefore an oil-based primer must go on first.
Before applying the primer, thoroughly wash the surface of the aluminum with a scrub brush and any phosphate-free soap. Rinse the aluminum well, until no more soap can be found in the run off water. Allow the aluminum to dry for several days.
Next, thin down the oil-based metal primer (which should be the same color as your top coat) with paint thinner at a ratio of one quart of paint thinner to every gallon of primer. Then apply the primer and allow it to dry.
Acrylic Latex
This coat should be applied within 48 hours of the primer. Before applying it, lightly sand the primer coat with 150 grit sandpaper to help the acrylic latex adhere to the primer.
The acrylic latex paint that you use should have the highest concentration of paint resin that you can find. The general rule of thumb is that the more expensive the paint, the higher the concentration.
Apply the paint on a cool, overcast day with a minimal amount of wind. The longer the paint takes to dry, the more securely it will bond with the primer.
Top Coat
For a professional look, finish off your paint job with a high-quality acrylic latex paint made for metal siding. Not only will it give your aluminum an enviable gloss, but it will make your paint job last much longer.