Masking fluid is a flexible latex adhesive applied to canvas or paper before watercolor paint. Once the paint is dry, the masking fluid can be rubbed away with a soft eraser or your fingers, revealing the blank canvas underneath. While many brands of masking fluid are commercially available, you can make your own formulation from any latex adhesive. Rubber cement also works well as a masking fluid, particularly on canvas.
Pour equal parts of fabric cement and water into the container.
Dip a wet brush into a water color paint, then into the container.
Repeat until the color of the masking fluid is satisfactorily dark. Otherwise, the masking fluid will be clear and difficult to see on the canvas.
Stir the mixture gently with the spoon. Do not shake, as it may cause bubbles and make the fluid difficult to apply.
Cover the container to store the masking fluid for future use.