Bondo is a putty commonly used for auto-body applications because of its ability to harden and sand to extremely smooth surfaces. Using Bondo to repair door surfaces achieves the same smooth results. Doors made of wood or metal can benefit from Bondo's filling ability. The putty comes in two parts that have to be mixed together and applied to the surface. A portion of Bondo is spread out over a hard glass or metal surface and hardener is mixed in. The mixture must be applied to the door immediately after mixing before it hardens.
Wipe the damaged area of door with a wet painter's rag to remove all grime and dust. Use paint thinner if the grime is oily or greasy. The surface must be cleaned before the Bondo is applied.
Sand the area with 120-grit sandpaper with side-to-side and up-and-down motions. This roughs up the area to create more surface area for the Bondo to adhere to. Wipe the sand dust away from the area with a painter's cloth dipped in paint thinner.
Place a blob of Bondo on the lid of a paint can by scooping it out of the Bondo tub with a 4-inch plastic putty knife. Squirt a line of hardener next to the Bondo and mix the two parts together until the Bondo is salmon-pink. It's better to add less than too much.
Press the Bondo mixture into the door dent, hole or crack, using the putty knife. This is the filling layer of Bondo used to just fill up the impressions. Repeat this process until the fill layers are just below the actual surface level of the door. Use up the Bondo as quickly as possible before it hardens.
Mix another small amount Bondo. Apply another layer in the impression to bring the level of the impression up with the level of the door. Use the putty knife to spread this layer. Avoid going over it after it begins to set up or you'll mar the Bondo surface. Allow it to set up and harden.
Mix one more small amount of Bondo and apply a feathered or thin layer over the impression and out above the actual surface level of the door. Keep this layer as smooth as possible. Allow it to harden.
Sand the Bondo with 120-grit sandpaper until the level of the impression is the same level as the surround surface area of the door. Don't sand all the way through the feather layer. Sand until it is perfectly smooth and then prime and paint the door.