How to Waterproof a Windowsill Well

Updated February 21, 2017

Windowsills are tilted at an angle to naturally shed water as it runs down the windows when it rains. However, the windowsill must be maintained regularly so that it remains waterproof. The painted finish and window caulking must be in good condition for the windowsill well to effectively shed water. Inspect your windowsills seasonally to avoid costly damage or repair work.

Pry out the old caulk line along the seams of the sides and back edge of the window sill with a putty knife. Score along both edges of the caulk with the blade of the putty knife and pry it out carefully to avoid damaging the wooden sill.

Scrape any loose paint on the windowsill away using a scraper. Further remove loose paint with a wire brush. Be sure to scrape and brush the sill in the same direction as the grain to avoid scratching the wood.

Sand the windowsill with a progression of medium and fine-grit sandpaper until it is completely smooth. Be sure to sand with the grain.

Insert a window caulk into the caulk gun. Cut off the tip at an angle. Apply a bead of caulk along the back seam and ends of the windowsill to replace the old caulk line. Run your finger along the bead to smooth it into the joints to create a seamless caulk line. Allow the caulk to cure.

Apply a primer coat to the windowsill and allow it to dry. Brush an exterior paint over the primer and allow it to dry. Add a second coat.


Inspect your home in the fall and spring to make sure your windowsills are in good condition.


Wear eye protection when scraping and sanding.

Things You'll Need

  • Putty knife
  • Scraper
  • Wire brush
  • Sandpaper
  • Window caulk
  • Caulk gun
  • Primer
  • Paint
  • Paintbrush
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About the Author

Jonah Morrissey has been writing for print and online publications since 2000. He began his career as a staff reporter/photographer for a weekly newspaper in upstate New York. Morrissey specializes in topics related to home-and-garden projects, green living and small business. He graduated from Saint Michael's College, earning a B.A. in political science with a minor in journalism and mass communications.