What do I need to do to restore a wooden entry door?
We just use good ol' bondo to fill missing pieces in wood. In the cabinets I did last weekend, several of the doors had an inch worth of corner rebuilt from bondo. 

The other tool that helped us that weekend was a profile detail sander; specifically the octosander by Skil. 

You can use the same spray gun you would use for cars, but you might need a larger diameter tip/needle combo. With my gun, I need a 1.7mm tip for oil based enamel and a 2.5 for water based. 

Edit to add our process: 


sand with an orbital sander and profile sander using 60 to 120 grit until the surface no longer has brush marks. It's OK if some of the original paint is still present. This ended up exposing some raw wood. Don't be afraid to press down hard on the sander at this point -- you're going for removal, not smoothness.
Clean areas that require patching or bondo using a stiff wire brush to remove any rotten or loose wood. Mix and apply bondo, keeping in mind that the set time of bondo is typically very small and that you want to apply it in it's most liquid state for it to bond well with the wood. Do not let it partially harden as you're applying it; stop and mix a new batch. Sand bondo after 45 minutes of hardening using a wood block with sandpaper wrapped around it so that you get a surface that is smooth with the rest of the door. 
Wipe down to remove sawdust and paint debris, wait for it to dry. Apply a thick coat of primer across the entire surface. Wait for it to cure (12 hours). 
Sand any areas that show as high LIGHTLY with 120 grit. This will probably include the bondoed area and any area around where raw wood was exposed. Sand more heavily with 120 grit in areas where the primer was attempting to fill in a crack. 
Wipe down, wait to dry, Prime again, lighter this time. 
Sand lightly with 220 grit until the surface is as smooth as a baby's bottom. You want to make sure you remove any signs of any orange peel or other imperfections in the primer coat, because they will show right through to the finish coat. 
Apply finish coat smoothly and evenly. If you're in a climate like mine (hot and humid), move the door indoors to cure for > 24 hours before attempting to mount and close it. 


Dad has always used exterior-grade Alkyd (Oil) based finishes on wood doors. Due to VOC restrictions, many of the finishes he used are no longer available, and the newer low-VOC formulations are prone to chalking. Our paint store, who I trust implicitly, now recommends that we use an alkyd primer and then coat with an exterior acrylic.