Keeping the external part of a wooden windowsill sound and free from rot is essential. Windowsills are designed to draw rainwater and other moisture away from the house but cannot do this properly if the windowsill is not protected by paint or varnish. Even a small patch of missing paint or varnish, will allow water to enter the wood and rapidly spread.
To repair a wooden windowsill which already has rot, first strip off any existing paint or varnish. If you’re using a hot air stripper, make sure you’re careful not to crack the glass. Any sections of the sill that are rotten should be cut away with a chisel or a knife until you reach the sound wood. It is now a good idea to paint all areas of the exposed wood with a good-quality preservative.
Use a good-quality exterior grade filler to fill any sections of wood that you have cut away. While you wait for this to dry you can check the damp proofing of the joint between the underside of the sill and the wall is sound. If it is not use a good-quality mastic to fill the joint.
Sand the dry filler flush, always working to the grain of the wood, and then prime the sill. Using exterior paint that matches the other sills on your house, paint the sills with one coat or two coats if needed.