Process

Normally I do not resort to using masking agents to preserve the white of the paper, preferring instead to paint around those areas.  But for this painting I needed to put in the sky in one flowing pass, so masking was the option.

When such a large part of the painting must be preserved, masking tape is the most efficient choice.
The most critical element is to choose a tape that is sticky enough to prevent leakage under the tape which would destroy the crisp edge that is desired.  Also make sure that the wash is completely dry before removing the tape.

I used scissors to form some of the shapes.  This may take a little time, but is well worth the effort.

Here's how the painting proceeded.  The scene is of Spring Ledge Breakwater Lighthouse near Portland, with Portland Head Light in the background.