The best way to paint aluminum is to clean it of chalk, mildew, dirt and other contaminants and then paint it with a good quality 100 percent latex paint. Aluminum that was previously painted may need to be cleaned with trisodium phosphate or primed before painting.
If when rubbing a finger over the paint, a transfer of white powder is visible on the finger, the trisodium phosphate is needed. This is because the paint has heavily weathered. Another option for the weathered surface is to use an oil-based or alkyd primer made for aluminum. If any bare areas of aluminum are found, they should be primed with a latex corrosion-inhibitive primer. Priming all of the aluminum instead of just the bare areas is not needed, but it can give a more even appearance when painted.
New aluminum that has been prepainted should be left to weather for at least six months before repainting. Sometimes the factory adds a wax or silicone finish that will affect the adhesion of paint if it's not weathered. A matte or flat type pf paint should be used on a surface that has dented. While satin or gloss finishes can look better at times, aluminum is susceptible to dents, and shinier paints may accentuate them.