Step 1
Clean the metal door with soapy water. Dry the door with a rag.
Step 2
Sand the metal door's repair area with 80-grit sandpaper. Remove all paint, primer and debris from the repair area.
Step 3
Place a golf ball-sized amount of the Bondo filler onto a piece of cardboard, using the manufacturer-supplied mixing trowel to scoop out the filler. A Bondo kit contains the filler, a hardening catalyst and the mixing trowel.
Step 4
Squeeze about a 3/4-inch-long bead of the hardening catalyst on the cardboard next to the Bondo filler.
Step 5
Combine the Bondo filler and hardening catalyst on the cardboard, using the mixing trowel to completely blend the two ingredients. Continue to mix the filler and catalyst until the Bondo has an even color.
Step 6
Press the Bondo mixture onto the metal door's patch area, using the mixing trowel to spread the Bondo. Feather the Bondo from the patch area out toward the metal door's surface. Let the Bondo dry for 30 minutes.
Step 7
Sand the dried Bondo's surface with the 80-grit sandpaper, using circular motions. Remove all imperfections in the Bondo patch's surface.
Step 8
Blend the edge of the Bondo patch into the metal door's surface with 180-grit sandpaper, working from the center of the patch toward the metal door with circular motions.