I'm thinking the door a fiberglass panel on the outside, but even if it is metal, and you can only powder coat metal, you would have to separate the outside skin from the rest of the door. Replacement doors are available if you want to spend the money. Trainjunkie posted a thread about a door replacement some time ago. My door is discolored but structurally sound and Barkeepers (try the dry powder it's better than the liquid) did improve it some, but some day, I'll probably paint
it. I checked on line and found a new door with lock for about $650.
A good automotive paint
store can color match and give you some paint
options for brush or spray applications.