Q. How do you keep paint from seeping under the edges of masking tape?
Brian Santos: The secret to keep this from happening is to heat-seal the tape.
Run a tapered plastic tool quickly over the applied edge of the blue masking tape after you've set the tape. This heats the edge of the tape, the waxy adhesive on the tape melts, and when it resolidifies at the edge, it creates a barrier that prevents paint from seeping underneath the tape.