The painting contractor should prepare and repaint the entire window unit. The painting contractor must remove all loose paint with a wire brush or scraper, this includes the ends and underside of the sill, as peeling or cracking often begins there. Then, finish the surface prep by removing all loose particles from the sill. Now the painting contractor must apply one coat of exterior wood primer, again paying special attention to the end grain and underside of the sill. After the primer dries, apply an acrylic latex caulk to the joints. The painting contractor must follow with one or two coats of a top-quality acrylic latex flat or satin house paint. Window sills are a perfect reason as to why you should take advantage of our home maintenance program. Every exterior painting job comes with the first year FREE. Horizontal surfaces are the hardest areas to keep up.