You must be aware of several factors before painting over aluminum. First, paint does not adhere easily to aluminum. Most companies prepare aluminum by applying coatings, making it easier for paint to stick. However, aluminum is still a tricky surface to paint over, and nothing is guaranteed to work. The second issue is that the surface may be exposed to extreme temperatures and weather, so a good quality primer and exterior latex paint is necessary to protect the surface for the long term.
Things You'll Need
Acid etch wash primer
Exterior latex paint
Remove all screens, if applicable, and you may want to remove the door entirely. Prepare the the door by rubbing sand paper to strip the surface. Remove all contaminants by cleaning with a sponge and water. You may also use a garden hose.
Apply wash primer with a paintbrush to the entire aluminum surface. Let dry for several hours.
Apply an exterior latex paint over the aluminum surface. Let dry overnight before replacing screens.
To avoid harmful fumes, be sure to paint in a well ventilated area.