Bondo is usually associated with car or fiberglass patching, but it makes an excellent filler for wood projects where you don' t want to replace the entire piece of wood. Clear out the damaged wood so it is free of rot, dampness and debris. Apply the Bondo into the crack or hole and smooth it as closely to the final form of the wood as possible. Sand off excess and fill with spackle if necessary.
Clean out the damaged area. Use a putty knife, screwdriver or chisel to remove any damaged wood. If the wood is damaged by wet rot, make sure all the affected wood has been removed. Do not apply any filler until the surrounding wood is completely dry.
Clear out all debris. Blow or vacuum all damaged wood out of the damaged area. Use a hair dryer or fan to completely dry out wood that is damp.
Mix the Bondo according to the instructions on the can. Note that you will have a short amount of time to work with the Bondo before it hardens. Note how long you will have and plan accordingly.
Spread the Bondo into the damaged area. Work it into the area so it fully contacts the damaged area and is as close to flush with the surface as possible. A little extra Bondo can be sanded down easily, but too much excess can be difficult to remove.
Sand the bondo flush. Use a sanding block or piece of wood to back the sandpaper for a flat finish. If there are spots in the bondo that are below flush, you can fill them with either more Bondo or with a standard wood spackle.