Homeowners can repair minor damage on windowsills with epoxy or polyester resins, states ThisOldHouse.com and RonHazelton.com. For surface damage, homeowners may only need to scrape off or sand down the damaged area and repaint.
Homeowners who have windowsills with dry rot or other water damage must remove the damaged area completely before repair, as remaining rot can continue to spread, according ThisOldHouse.com. Homeowners should first cut out and remove all of the damaged areas, scrape off any excess paint, and then fill the gaps with an epoxy or polyester resin to match the existing casing. The homeowner then finishes the project by painting the sill.