A split, cracked or simply weathered window sill can detract from an otherwise beautiful window. The sill--or lower horizontal wood plate--extends into the room creating a target for impacts and other damaging factors. Repairing weathered sill finishes is simply a matter of sanding and repainting. Other sill repairs can extend from basic maintenance to reconstruction of the sill. Knowing what common tools and materials to use is the first step to a successful repair.
Scrape the sill with a metal scraper to remove chipped or peeling paint. Sand the sill with 100 grit paper to smooth the surface for priming. Use a random orbit sander for the best results.
Prime the sill with a coat of oil-based binding primer. Use a disposable brush and apply the primer as evenly as possible to prevent runs and drips. Cover the entire surface, giving special attention to areas where the paint was worn through. Allow the primer to dry.
Apply two coats of semigloss latex paint in a matching colour with a soft bristle brush. Allow the recommended drying time between coats. Use long, even strokes in the direction of the grain.
Fill any chipped or dinged areas with auto body filler putty. Mix 10 parts of putty to one part activator with a flexible metal putty knife. Mix only what you can use in five minutes or less. Apply the putty to fill the damaged areas, smoothing it off even with the sill's surface. Allow the putty to harden for 15 minutes.
Sand the putty down smooth with the surface of the sill. Use a random orbit sander with 100 grit paper for best results.
Apply primer and paint, as outlined for weathered sill repair.
Realign the split pieces to determine if there is any missing material. Small gaps can be repaired as with chips and dings (see Section 2). Larger missing pieces may be beyond repair, requiring complete window sill replacement.
Apply wood glue to the inside surfaces of the split. Realign the pieces and drive finish nails through the edge of the sill with a hammer to hold the split in place as the glue hardens. Allow the glue to dry completely.
Fill any remaining gaps with auto body filler, allow it to harden and sand the surface smooth with 100 grit paper. Prime and paint the sill.
Hammer up from the bottom of the sill to loosen the nails. Use a flat pry bar to lift the sill until the nails release. Remove the sill from the frame.
Use the old sill as a template to mark out a replacement on a piece of 1-inch thick pine lumber. Mark the outline as close as possible to the original.
Cut out the new sill with a jigsaw, staying as close to the lines as possible. Fasten the pine to a sturdy work surface with a C-clamp to make cutting easier.
Use a rubber mallet to tap the new sill into place and nail it to the sub-sill underneath with finish nails and a hammer. Sand, paint and prime the new sill as outlined in the previous sections.