Remove the aluminum closet door and lay it down on a flat surface, with the exterior side of the door facing up. Depending on where you are painting, you may want to lay drop cloths under the door.
Wipe the closet door off with an abrasive sponge and water to remove dust.
Dry the closet door with a dry, lint-free cloth.
Paint the closet door with acrylic primer paint. Use even strokes and paint in one direction. You will only need one coat.
Allow the acrylic primer paint to dry.
Paint the closet door with acrylic topcoat paint. Allow the paint to dry, then apply a second coat.
Allow the acrylic topcoat paint to dry and then reinstall the door.