Over time the wood of the windowsill may weaken. It can become cracked, and even begin to break apart. If the condition is too far gone, the windowsill can have the bad wood removed and replaced with new wood. Sometimes, though, a windowsill is in need of lesser repairs. These repairs include nailing, hole repair and repainting.
Determine whether or not the window sill needs to be replaced. If the wood is cracked or rotten, then your best bet is to remove the old wood and replace it. Do this using a claw hammer. Pull up the old sill and dispose of it. Measure the length and the width of the sill. Cut a piece of any hardwood to those measurements. Lay the hardwood in place and secure using plasterboard screws and a drill. Indent the screws slightly.
Tap in any nails that are sticking out. If the sill board is loose, secure it by hammering in a few new nails. Gently tap 2.5 cm (1 inch) nails into the sill in the loose areas. If the small nails don't seem to sink into anything, you may need to use longer screws and the drill.
Use plasterboard filler to fill in any cracks or crevices, holes or scratches that may flaw the sill. Smooth over flaws evenly. Allow the filler to dry.
Sand the sill completely. Sand over the areas of filler to smooth them, and then sand the entire sill surface. This will smooth the sill surface and get it ready to be painted. Once you are done sanding the sill smooth, remove all dust with a damp cloth.
Use masking tape to tape around the edges of the windowsill. Lay down a painter's tarpaulin or piece of plastic to protect the area beneath from paint. Use a small paint brush to paint the entire surface of the sill. Allow to dry. Repeat. Remove masking tape from the edges. Clean up any mess. Allow windowsill to dry for 24 hours before placing anything on it. Enjoy the fresh new look your old sill has.
Keeping window sills painted helps protect the wood.
Use care and caution when using a drill, a saw or any other power tool to avoid injury.