The best tape for creating straight edges is “Watercolor Washout Tape” which looks like a big roll of cellophane tape but it’s thicker and stronger. It costs more than the blue painter’s tape from the hardware store but gives perfect edges if used correctly. Using correctly means making sure the tape is secure on the paper by smoothing with a burnishing tool or finger over the tape. One way to get a double use from the tape is to only put down half a side. After it’s taken off the paper the tape can be flipped around and the clean edge can be put down.