How to Repair Exterior Window Sills

Written by timothy burns
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Window sills are one of the few areas on a home where wood trim sets horizontally. Because their flat surface catch water and snow, window sills receive a higher degree of weather damages than vertically aligned siding and trim and may need to be repaired more often. By following this multi-step procedure, wood sills can be restored when damaged by wind, rain, insects and weather.

Skill level:

Things you need

  • Bondo wood filler
  • Rotted wood stabiliser
  • Small tip screw driver
  • Sand paper, rough and fine grade
  • Stiff 1-1/2 inch putty knife
  • Five-way painter's tool
  • Exterior alkyd oil primer
  • 2-inch paintbrush
  • Paint thinner
  • Household rubber gloves
  • Siliconized acrylic caulk and caulk gun

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  1. 1

    Scrape and remove all loose or peeling paint and caulk with the putty knife. Using the pointed end of the painter's tool or screw driver tip, remove any damaged or decaying wood on or below the surface.

  2. 2

    Apply a rotted-wood stabiliser to the area where the damaged wood was removed. This product hardens the area around where the wood was removed and prevents any further rot. Allow to harden according to manufacturer's instructions.

  3. 3

    Put on gloves. Mix Bondo according to manufacturer's instructions. Press Bondo into the damaged area of the wood sill. Slightly overfill the damaged portion of the sill. Allow to cure according instructions. Bondo cures chemically as opposed to drying. The product will become warm, and stiffen rapidly as the curing process begins. If the repair is not complete and the Bondo begins to change consistency as described, it is better to throw away the unused portion, and mix a new batch rather than try to work with patching material that is hardening.

  4. 4

    Sand the Bondo to a smooth surface. Start with the rough-grade sandpaper to remove any excess material. Sand with the fine-grade sandpaper to prepare for priming.

  5. 5

    Apply alkyd primer to the entire window sill. Allow to dry according to manufacturer's instructions. Apply caulk around the perimeter of the sill, at any point where the sill meets adjacent surfaces to prevent water penetration.

  6. 6

    Paint sill with finish paint to match existing trim.

Tips and warnings

  • Make sure to allow time for each product used in this process to completely dry or cure before applying the next material. Failure to do so will often create a failed repair, requiring that the entire process be repeated.

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