Things You'll Need
Orbital sander or grinder and metal sanding disc
Steel wool or metal sandpaper
Rags and towels
Sandpaper, 200 grit or higher
Metal doors often rust when water infiltrates the small cracks in the paint and contacts the metal. This causes a blemish on the door and may lead to corrosion, which weakens the door structure. Repairing this issue requires a refinishing of the door. So, if you are handy and good at painting, this project will be fairly straightforward. It is complex, and many steps must be taken to ensure the fix is a permanent one, but with some effort you'll repair that rusted door in no time.
Remove the rust and paint on the entire side of the door where the rust is located. First, scrape the rust and paint off with a straight putty knife. Sand away remaining rust with a power sander, or a power grinder with a grinding disc for metal surfaces.
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Remove remaining smaller patches of paint and rust by hand sanding with steel wool or sandpaper. Continue until all the rust and paint are removed from the door surface. Wipe the entire surface clean with a rag or towel.
Prime the door by spraying on a coat of metal primer. Apply the primer to the entire surface of the door. Let the door dry for as long as the instructions on the primer recommend. Usually, this is between four and 24 hours.
Apply a top coat of exterior metal paint to the surface of the door. Use a spray paint can of high quality exterior paint if you want this application to last longer.
Let the top coat of paint dry as indicated on the instructions. Sand the top coat lightly with 200 grit or higher sand paper, wipe away the residue, and apply another coat of paint for a smooth finish.