Masking Fluid is used to mask areas of the paper making them resistant to watercolour. Sections treated with Masking Fluid must be allowed to dry before overpainting. Once dry these areas remain protected and cannot be penetrated by colour. It can be applied to white paper or to areas previously coloured. As it is latex based. Masking Fluid is best applied with an old brush or pen. Masking Fluid should not be diluted before use or used on damp or soft sized papers. Remove as soon as possible once the painted area is dry, with an eraser.