Hi, Anthony. There are a couple of factors that go into this decision. If the paint that is on there now is chipping or flaking, you will want to feather the painted areas into the unpainted areas with sandpaper and prime. If you are changing from a dark color to a light color, you will want to prime. And particularly with a door, because it is subject to getting oily and dirty, I would wash with Tri-Sodium Phosphate and rinse well before sanding. If your present paint is in otherwise good condition, you can probably get by with just washing, sanding and repainting.