Although wood is sturdy, it isn't impenetrable. Exterior wood window sills are regularly exposed to outdoor elements, such as rain, hail and ice, which damage them over time. Water permeates exterior wood and causes window sills to develop dry rot. Decay spreads through window sills until they become structurally compromised. Restore a rotted sill promptly, before it deteriorates further and breaks loose from the window. You can repair a wood window sill with water damage using certain techniques and products.
Extract the rotted wood from the window sill. Using a cold chisel, dig out the soft, loose wood until only dense wood remains.
Drill 1/4-inch-deep depressions into the solid portion of the gouged wood.
Seal the gouged window sill with liquid epoxy consolidant (also called a penetrant) to prepare a durable foundation for the epoxy filler. Using a paintbrush, apply two coats of the liquid epoxy to the damaged wood. Allow each coat of liquid epoxy to dry separately for 5 minutes.
Prepare a batch of epoxy wood filler. Using a putty knife, blend the two-part epoxy filler according to the manufacturer's directions.
Fill the hollowed-out portion of the window sill with the epoxy wood filler. Pack the epoxy filler into the gouged wood using the putty knife. Mold the filler to the sill's contours. Let the filler cure according to the label's instructions.
Sand the entire patched wood section with medium-grit sandpaper. Sand off the excess epoxy filler until the patch becomes flush with the window sill.
Wipe all sandpaper dust off the sill using a dry cloth.
Prime the window sill with an exterior oil-based primer. Using the paintbrush, apply an even coat of primer over the wood surface. Let the coat of primer dry as recommended by the manufacturer.
Brush two even coats of exterior latex paint on the entire window sill. Apply the paint using smooth, even strokes. Allow each coat of paint to dry separately as advised by the manufacturer.
Have a professional contractor remove and replace the window sill if it is thoroughly rotted.