Winsor & Newton produces these mediums to the same exacting standards as all of its artists' materials. Masking fluid is used to mask specific areas of the paper, making them resistant to watercolours. Winsor & Newton produce 3 types of masking fluid for use with watercolours.
Art Masking Fluid - Coloured: This masking fluid can be applied to white paper or to previously coloured areas and has a slight yellow tint to show where it has been applied. As it is latex-based, it is best applied with an old brush or better still a quill or nib as the latex will peel off. Art masking fluid should not be diluted before use or used on damp or soft sized papers. The dry fluid can be removed by rubbing with an eraser.
Art Masking Fluid - Colourless: The same as above, but a colourless version.
Permanent Masking Medium: This medium can be applied direct to the paper or mixed with watercolours first. It is ideal for isolating areas of fine detail, but areas treated with Winsor & Newton Permanent Masking Medium must be allowed to dry before overpainting. Once dry, these areas remain protected and cannot be fully penetrated by further washes.