If you have an older home and have had estimates for replacing old, rotted-out window and door sills, you know how expensive it can be. Bondo, the same product used as auto-body epoxy filler, has been used by handymen for years to repair damaged wood. In fact, it works so well that the makers of Bondo now sell a similar epoxy product just for wood. If you have old, worn out, even rotting woodwork on your home, and you want to save money on expensive replacement costs, try Bondo.
Scrape out as much of the crumbling, loose wood as possible. Let it dry out. You can use a fan or blow dryer if you need to, but the area has to be dry for the Bondo to stick.
Fill the freezer bag with the Bondo mixture, then snip off a corner of the bag and use it to squeeze the Bondo into tight areas. Paintbrushes and putty knives can be used to mold and form the material as best you can. Work quickly because it dries hard in 10-15 minutes. If the repair is very deep you may want to do two coats. Do the best you can to get the repair to conform to the shape you want, to minimize a lot of arduous sanding later on. Bondo is hard to sand once dry.
Sand the Bondo. A palm sander allows you to work edges and finesse the sander to achieve the profile you want. Start off with 100-grit sandpaper for rough areas, but finish the job with 200-grit paper, otherwise you will scratch the surface.
Paint two coats with your chosen paint over the repair, and you're finished. Bondo requires no special primer, it is impervious to rust and moisture.