Repeatedly using a brush to apply Art Masking Fluid will damage the brush over time.
Although some artists swear by vaseline, soap or petrol, our tests do not show any of these methods to be 100% reliable or safe. We suggest that if you are using a brush, you make sure the masking fluid used is as fresh as possible and that you wash it from your brush with soap within a split second of use.
It is better to use low-priced or old brushes if possible. If fine lines are required, a goose quill is ideal and for larger areas you can try a colour shaper (a colour shaper is like a brush, but instead of hair it holds a rubber shape). Both are excellent for this purpose, because their lack of hair ensures the fluid can be rubbed off easily.