Many months ago, I came across an art medium used by painters. I cannot remember the exact term for this medium. It is white, but is transparent once used, I believe... having the consistency and appearance very similar to white liquid Elmer's Glue.
In essence (referring to this medium as Medium A), a painter would first sketch their desired image in pencil onto paper... be it typography or any image, really. Any spaces they wanted to remain white in the finished work (like lettering or small details, for example), they would paint and fill in with Medium A. Once medium A has dried, they would paint all over the paper (including over Medium-A-covered areas) with watercolor, typically with multiple colors. After the painting has dried, the artist would peel off Medium A areas from the painting, being careful not to rip up the paper fibers themselves along with Medium A. In essence, it has a resistance effect similar to a white wax crayon drawn underneath watercoloring.
An example of what the result after using Medium A could look like:
What is Medium A really referred to as?