Sketch your desired picture or design on the paper using a pencil. Watercolor paint is translucent, so make the marks light enough so that you do not have to erase them after painting.
Shake the masking fluid to help combine the mixture. Many masking fluids come with a brush or sponge applicator, but if your masking fluid does not, use a wet small, thin and disposable paintbrush. Masking fluid pens are also available.
Brush the masking fluid onto completely dry paper, applying it as you would paint. If you have already painted on the paper and want to use the masking fluid to keep the paint color the same while applying another color, make sure the paint is dry first.
Let the masking fluid dry. See the bottle of fluid for a precise drying time, usually from 30 minutes to 1 hour. Even when dry, it will be a bit tacky to the touch.
Paint the area as desired. Masking fluid is similar to painters’ tape on a wall; you can ignore it and paint over it with your main color, and the space underneath it will remain unpainted. Set the painting for aside for 1 hour to make sure the paint dries thoroughly.
Remove the masking fluid using a rubber cement remover or plastic eraser. Rub the area with the masking fluid in one direction, using the corner of the eraser. The masking fluid will come off in balled-up pieces; remove them from your painting with your hand. The area is now ready for the proper paint color.