Masking fluid is a liquid used to block out areas of a watercolor while you paint, thereby retaining the white of the paper or the previous color that was painted. It's a solution of latex in ammonia and is removed by gently rubbing it off either with your fingers or an eraser, once the painting is dry.
Winsor and Newton Art Masking Fluid 75ml How is it used? Masking Fluid protects areas of your work when colour is applied in broad washes. Masking Fluid works by direct application to the paper. It can be applied to white paper or previously coloured areas.Follow these helpful tips to get the best results when masking. Shake bottle before use. Do not use on wet or damp paper. Use gelatine surface sized paper, this helps to 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. Do not leave fluid on paper for long periods of time.