From Cathy Johnson, in the April 1984 issue of The Artist’s Magazine:
Ordinary masking tape may be used to keep small areas white, mask horizon lines when painting juicy skies or produce a strong directional edge. First, place tape over the area to be protected. Using a sharp X-Acto knife, cut through the tape only to the paper’s surface. Gently pull off the outside shape of tape, being careful not to tear the surface of your paper. Paint over the tape and allow the wash to dry before removing the remaining tape.