Tips for using different watercolour art masking fluids
When you want the freedom to apply broad colour washes across your painting surface but do not want the wash to cover key details within a composition, Art Masking Fluid can help. 
 
Apply Art Masking Fluid directly to dry paper. It can be used on white paper or previously coloured areas, and it acts like skin over those areas you want to protect from the colour wash. Once you have finished applying the colour wash, simply remove the masking fluid to reveal the protected area underneath. 
 
Art Masking Fluid has a slight yellow tint, making it easier to see where it has been used. 
 
Here are some helpful tips to get the best results from both types: 
Shake the bottle before use. 
Do not use on wet or damp paper. 
Use gelatin surface-sized paper; this helps prevent the fluid from adhering too strongly to the paper. 
Use old brushes or dip pens to avoid damaging valuable brushes. 
Wash brushes in soapy water immediately after use. 
Ensure fluid is dry before applying colour wash. 
Do not leave fluid on paper for long periods of time. 
If spilt by accident, wash item immediately in soapy water. Once dry, the latex can only be removed by picking at it or rubbing with an eraser. Unfortunately, there are no solvents available to remove it. 
 
 
Here's a great video to watch
https://www.youtube.com/watch?v=7FEOCP4XBzE