In this video, This Old House general contractor Tom Silva saves a rotting window with a two-part epoxy filler.
1. Use trim router or rotary cut-out tool fitted with a V-shaped bit to grind out all the rotted wood. Stop when you reach sound, solid wood.
2. Use a paint scraper to scrape off any old paint.
3. Mask off the area around the repair with painter's tape.
4. Take a narrow paintbrush and apply an epoxy bonding agent to all of the exposed wood.
5. Hold a 4-inch-wide plastic putty knife against the bottom end of the casing, and trace the casing's profile onto the blade of the putty knife.
6. Cut the casing's profile from the knife using aviation snips.
7. Squeeze some epoxy onto a Lexan panel and thoroughly mix it with a putty knife.
8. Force the epoxy into the cavities where the rotted wood was removed.
9. Smooth the epoxy with a putty knife.
10. Hold a straightedge against the underside of the windowsill, then strike off the epoxy with a putty knife to fill any remaining voids and create a perfectly straight line.
11. Use the putty knife that was trimmed with the aviation snips to shape the epoxy to match the original casing.
12. Allow the epoxy to cure overnight, then sand it smooth and apply one coat of primer and two topcoats of paint.