Using a plastic paint scraper tuck the painters tape down beside the carpet all along the trim. Continue to the lower sash beginning with the crossbar. 7 Ways To Completely Rethink Your Window Sills Trim Paint Color Painting Trim Decor Dont forget to tape above the trim also. How to paint window sills . Luckily even if dried paint can easily be scraped from the glass with the right tools. Allow 2 or 3 hours for the primer coat to dry. Now you have and there is no ignoring the rotten window sill. If you close the window while paint is still wet the window. Finally paint the window casing and the window sills then let the paint dry. Work the white spirit into the brush and then wipe away the paint with some Seriously Good Paper Towels. For large flat areas such as the shelf of the window sill use the Taskmaster Gloss 4 inch Roller. The other is to use a. Sand until smooth and level. Once your windows are clean and dry youre ready to prime. Step 3 Apply two ...