Masking fluid in watercolour painting can be both a blessing and a curse. It can allow you the freedom of slopping down wet paint and water without worry. What most people that have used it will tell you is that it is also a royal pain to apply. Akin to painting with watered down bread dough it gets gummy and clogs up and can ruin a good brush.
A few tips...
- I only ever use synthetic brushes, The bristles are stronger than real hair and take more abuse.
- I always coat my brush with brush soap before dipping it in the masking fluid. If I am using it a lot in a particular painting I will clean it off and repeat this as needed.
- Clean your brush right away and re apply a leave on brush soap. ( think of it as a leave in conditioner )
When you remove it you will be left with stark paper white areas. Often it looks terrible because of the lack of precision you applied the masking fluid do to it being so difficult to apply. You should not think of this as the final stage, light washes of paint can be applied to reduce the stark contrast if need be.