Artist 
Writer Masking fluid
Masking fluid (or frisket) is a rubber latex solution used to mask out areas on paper, used in watercolor painting, and so you retain the white of the paper or the previous color that was painted. You can remove it by gently rubbing it off either with your fingers or an eraser, once the painting is dry. You must wash your brush immediately after using it to apply masking fluid or it will be ruined.