Masking Fluid from MM WaterColors is a paintable and later peelable rubber liquid from the sheet, also known as Masking Fluid or Drawing Gum.
 
Lightly colored green, it is easily visible on white backgrounds.
 
It can be applied with a brush or pen to the areas of the image to be masked. However, the silicone brush is best suited.
It can be painted over after it is completely dry. 
To expose masked areas, peel or rub away with finger after paint is completely dry.
 
Clean normal brushes with water immediately after use.