Three things are needed to paint a razor sharp edge. 1. Tape, 2. Clear latex caulk, and 3. Laser level. First position the laser level to project your edge line. Next tape on the laser line. Any paper type masking tape works even on rough surfaces. Then apply clear latex caulk on the edge of the tape over lapping slightly (approx 1/4 inch) the side of the tape you want to paint. Now paint immediately while the caulk is still wet. Peel away the tape as soon as its painted. Razor sharp every time.