If your dog or puppy has gotten a little hyper and chewed your windowsill, you can sometimes repair the damage yourself with a few tools. Repairing chew marks is done in a manner similar to fixing any other type of windowsill damage, such as windowsill rot. If the damage to the windowsill is extensive, and large sections are missing, you will have to replace the entire windowsill instead of simply repairing a few chew marks.
Clean the chewed areas of the windowsill. Use a stiff-bristled brush to knock away any loose wood. Use a screwdriver to lightly dig out any larger splintered wood.
Go over the area where the dog chewed with 200-grit sandpaper, making it as smooth as possible.
Apply liquid wood hardener, available at most home-improvement stores, to the damaged areas of the wood, if needed, using a paintbrush. Follow the manufacturer's instructions.
Build up the damaged part of the sill, using small chunks or blocks of wood, if sections of the sill are missing and there's enough sill left to support new wood. Cut the blocks to fit the shape of the sill with a handsaw. Screw the new wood into the sill. Sand the blocks using an orbital sander and sandpaper to shape the wood to the sill.
Mix a batch of epoxy primer per the manufacturer's instructions. Apply the primer to the damaged wood, using a paintbrush. Wait for the primer to dry.
Blend epoxy filler per the manufacturer's instructions until the mixture is smooth and consistent. Apply the filler to the surface of the sill with a putty knife, filling any holes or gaps as you work. Shape the epoxy with the putty knife until the repaired area looks the same as the rest of the windowsill. Let the epoxy dry and harden completely.
Sand the surface of the windowsill smooth. Apply a second coat of epoxy if needed and sand again. Paint the windowsill.