It is important to prepare a metal door before painting it. Use a heavy grit sandpaper to rough the metal. The sandpaper makes the metal have a rough texture or surface instead of the slick, shiny surface. The paint will adhere to a rough surface better than a slick surface.
First, sand with coarse paper. Second, prime with Rustoleum or a similar product. Finally, apply a Latex finish coat, which will expand and contract better with the door when the temperature changes.