Painting a steel door is somewhat like painting a car — the quality of the project depends on a large extent on how much effort you put into preparation. You need to fill and flatten dents, or else they will appear, and you’ve got to clean, etch and prime the surface, or the paint might lift. You get the best results using oil-based paint with a sprayer, but should youn’t have spray gear, you’ll get excellent results with a brush. The important thing is to thin the paint so it flows out and also to propagate it in thin coats.
Remove all the door hardware, for example, knob, deadbolt and latch, with a screwdriver. Cover the hinges with masking tape and also the windows with masking tape and paper. If the door has weatherstripping, pull it off and wash the adhesive off the door with acetone on a rag. Unscrew and remove the sweep on the bottom of the door, if there is one.
Wash the door with soap and warm water. After the door dries, wipe it down with a rag dampened with acetone to remove any grease that the detergent did not dissolve.
Fill dents, depressions and other defects with auto body filler. Mix the filler with the hardener that comes with it at the proportion recommended on the container and then spread it where required with a putty knife. Scrape it flat with the knife before it hardens. When it hardens, sand it flat with 120-grit sandpaper, then sand the entire door with 220-grit sandpaper to etch the old finish. Wipe the door with a rag.
Prime the door with an oil-based metal primer. Spray the primer with a compressed air spray gun or disperse it with a brush. If you spray, keep the door open and then cover the door with vinyl sheeting. Put on a respirator while spraying or painting with a brush.
Let the primer dry completely, which requires about 2 hours, then sand the door lightly with 220-grit sandpaper and wipe the dust off.
Spread one thin coat of oil-based paint by spraying by painting it with a brush. Whether you wax or spray, thin it with about 10 percent mineral spirits to help it flow out. Do not worry should youn’t get complete coverage with this coat.
Let the paint dry overnight, then sand it with 220-grit sandpaper and apply another thin coat. You might be satisfied after this coat, but otherwise, apply a third coat after sanding the second.