Eliminate The Trim Gap For A More Finished Appearance. Installing traditional flat trim over a window nailing flange usually leaves you with a wider gap to caulk. Kleer Trim Boards with Nailing Flange Cutouts give you ¼ in. space to overlap most window flanges to let your trim piece lay flat against the wall. That means it’s easier to create tighter corner joints and a more finished look around your window framing and walls.