All doors suffer a certain amount of abuse from being slammed, kicked and scraped. Exterior doors must also withstand the deteriorating effects of sun and moisture. When cracks and holes appear in your doors, as they inevitably will, patch them with one of the wide variety of wood fillers available from hardware and home-improvement stores. These range from lightweight spackling compounds to two-part epoxies that are harder than actual wood. Whichever you use, plan ahead so you use a material that will blend in with the door, and be prepared to spend some time sanding.
Fill small scrapes, cracks and gouges in the door with spackling compound. This is a non-setting, lightweight filler normally used for patching small holes in walls. Spread it into the defect with a putty knife and let it dry for about 30 minutes, then sand it lightly with 120-grit sandpaper. Spackling compound is white, so you'll have to touch up the paint on the door after you use it.
Fill holes larger than 1/4 inch in diameter with patching compound. This type of filler will set to create a rock-hard substrate for a surface patch. Mix the powdered compound with water in a paper cup, following manufacturer instructions, until it is mouldable. Work it into the holes with a putty knife and let it set for an hour. Spread spackling compound over it and sand the spackling compound flat when it dries. Touch up with paint as necessary.
Patch defects around the edges of the door, the knob or the hinges with two-part epoxy filler. This type of filler comes in different colours to match different woods. Mix the filler with hardener according to the instructions on the package, then work it into the hole or crack with a putty knife. Allow it to stiffen for about 30 minutes, then shape it to match the door's contours. Let it harden overnight, then sand it as needed with 120-grit sandpaper.
Patch extensive cracks and splitting in exterior doors caused by sun exposure with exterior spackling compound. Like the interior variety, this is usually white, but it remains flexible and won't shrink after it dries. Spread it into the cracks with a putty knife, let it dry overnight, then sand it and repaint the door.
Fix a large hole in a hollow-core door with a block of wood, followed with patching and spackling compounds. Find a block of wood that has almost the same thickness as the space between the two faces of the door and that is just slightly smaller in diameter than the existing hole. Spread one side with carpenter's glue, then fit it into the hole and press so that it adheres to the inside of the face opposite the hole. When the glue dries, fill the hole with patching compound. Apply a topcoat of spackling compound to the hole. Sand and repaint as needed.
Use automobile body filler in place of 2-part epoxy wood filler for repairs that require strength. It dries white or grey, so paint must be touched-up. Fill small cracks in stained wood doors with flexible wood putty, available in colours to match various stains.