A window sill is normally constructed of a wood base that is either waterproofed with a sealant or painted a certain colour. Many paint manufactures sell waterproofed paint that will create a seal to shed away water as it hits the surface. Preparing your wood surface is critical to creating a strong bond for the sealer and paint. Prior to applying waterproofing paint you should sand the surface smooth apply sanding sealer and then a final coat of paint. Some window sills may require a layer of silicon coating between the sill and window frame.
Prepare the surface by removing any old paint. Apply the paint stripper to the window sill as per manufacturer's guidelines. Use the scraper to remove the paint. This may take several reapplications.
Sand the wood first with the 80-grit paper followed by the 120-grit paper to smooth out the surface. Never sand against the grain of the wood. Wipe the window sill free of dust.
Apply a coat of sanding sealer with a paint brush evenly along the window sill. When the first coat dries, lightly sand the windows sill with the 120-grit paper to remove air bubbles and drip marks. Apply a second coat and allow it to dry. The sanding sealer will waterproof and protect the wood. It also gives it a glossy finish.
Apply the waterproofing paint or oil base paint to the surface of the window sill using a paint brush. Again, sand the surface with the 120-grit paper in between the first and final coat. This will give you a smooth surface.
Open the silicon caulking and place it inside the caulk gun. Apply caulking to the joint between the window frame and the window sill. Do this after the paint completely dries and cures. Curing times will vary based on product. This is only necessary if you have a window frame lip that runs across the window sill that is designed to seal the window shut.