The Right Stuff
Sand the dented area down to bare metal with 150-grit sandpaper. Be sure to remove paint from deep crevices.
Don't repair a dented door with spackling paste or wood putty; neither compound bonds very well to metal. Instead, use an auto-body filler, which is specifically formulated for adhering to metal surfaces. This type of filler also dries rock hard so it sands smooth and flat, and it produces a perfect surface for paint.
For our door repair, we used the Bondo Auto-Body Repair Kit ($6), which includes a 15-oz. can of polyester resin-and-fiberglass filler, a small tube of cream hardener and a plastic spreader. You can find the kit at most hardware stores, home centers and auto-supply shops.
To complete the repair, you'll also need a rubber sanding block, a medium-size household sponge, acetone, two sheets of 80-grit aluminum-oxide sandpaper, two sheets of 150-grit aluminum-oxide sand-paper, one sheet of 400-grit silicon-carbide wet/dry sandpaper, metal primer and paint that matches the color of the door.
Auto-body filler must be applied to bare metal, so begin by sanding the area around the dent with 150-grit sandpaper wrapped around the sanding block. Then, remove the sandpaper from the sanding block and use it to hand-sand the deep recesses of the dent. Lightly wipe down the entire area with a clean cloth dampened with acetone or rubbing alcohol.
Auto-body filler is a two-part resin that must be mixed with hardener before it can be applied. Keep in mind, though, once the filler is mixed, you've only got about four minutes to fill the dent before the resin begins to harden.
Using a clean putty knife, scoop out approximately 8 tbs. of the gray filler, placing it onto a piece of plywood or cardboard. After gently kneading the tube of red hardener, squeeze out about 1/2 tsp. next to the filler. Thoroughly mix the two components with a putty knife for no more than one minute. Be sure that the filler is uniformly pink, with no gray or red streaks.
Next, scoop up some of the filler with the plastic spreader and smear it across the dent, forcing it deep into the crevice. Overfill the dent slightly and then feather out the filler beyond the dent by an inch or so all around. Note that large, deep dents will require two applications of filler.
Wait about 30 minutes, then check to make sure the filler is fully cured. (If you can scratch it with your fingernail, wait a bit longer.) While wearing a dust mask, begin smoothing out the filler with 80-grit sandpaper on the rubber sanding block. Continue sanding until all the high spots are removed and the filler begins to blend in with the surrounding surface. Switch to 150-grit sandpaper and finish-sand the entire patch.
If you find any voids or low spots, mix up another batch of filler and spread a thin coat across the surface. When this coat has fully cured, lightly sand the surface with 150-grit sandpaper.