Use these removable fluids to make specific areas water resistant. They will retain areas of white paper or areas of underpainting, then easily rub off with a finger or eraser. The handy dispensing bottle allows for fine detail work. - Schmincke Aqua Masking Fluid - Blue, 19 ml BottleSchmincke Aqua Medium, BLU MASK FLUID 19ML - Blue tinted Aqua Masking Fluid is the same formulation as Aqua Colorless Masking Fluid. It will retain areas of white paper or areas of underpainting, then easily rubs off with finger or eraser. The handy dispensing bottle allows for fine detail work. Dry the surface thoroughly after applying and before painting with watercolours. Do not leave masking fluid longer than two days on the paper. It is also recommended for airbrush work.