Found this (below)on the Winsor and Newton website. I bought some many years ago and have only used it a couple of times. I did not find it very useful for my work. You can buy it from SAA.
Permanent Masking Medium can be applied directly to white paper, to dried washes on the paper or mixed with water colours first. All water colour washes mixed with Permanent Masking Medium remain open and workable whilst the wash is still wet. Once dry, the area becomes isolated and cannot be fully penetrated by further washes. A hair dryer can be used to speed the drying. Brushes should be washed in warm water and soap before using other colours.
How does Permanent Masking Medium differ from Art Masking Fluid?
•Permanent Masking Medium is not removable
•Permanent Masking Medium can be added to colour
•Brushes are easier to clean
•Art Masking Fluid is recommended for crisp whites, larger areas of masking and is faster drying whilst Permanent Masking Medium is recommended for fine detail and expressive effects within the painting.