Fill a dent or hole in a steel door the same way a body shop would fix your car. You can do this with the door in place, but it will be easier with the door lying flat on sawhorses. Remove an area of paint a couple of inches larger than the damaged spot (Photo 1). Sand away the paint with 60- or 80-grit paper, or do the job faster with a small wire wheel in a drill.
Tip: If the damage is near the bottom of the door, you can skip the repair and cover it with a metal kick plate (find at home centers and hardware stores). Kick plates are about 8 in. wide and come in lengths to match standard doors.
Next, fill the wound with auto body filler (find at hardware stores and home centers). To mix the filler, place a scoop of resin on a scrap of plywood or hardboard. Then add the hardener. Mix the two components thoroughly; unmixed resin won't harden and you'll be left with a sticky mess. A plastic putty knife makes a good mixing tool.
Apply the filler with a metal putty knife that's wider than the damaged spot (Photo 2). The filler will start to harden in just a couple of minutes, so you have to work fast. Fill the repair flush with the surrounding surface. Don't overfill it and don't try to smooth out imperfections after the filler begins to harden. Adding another coat of filler is easier than sanding off humps.
When the filler has hardened completely (about 30 minutes), sand it smooth (Photo 3). After priming the repair, you could paint over the primer only. But the new paint won't perfectly match the older paint, so it's best to repaint the entire door.