Many painters use this watercolor technique to preserve white areas on the painting. There's much more you can do with masking fluid (aka liquid frisket)
Comparing Masking Fluids - Jackson's Art Blog
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Masking Fluids and Liquid Friskets
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Schmincke - Liquid frisket Coloured, 20 ml, 50 303032, colourless Masking Liquid for Opaque Areas of Paintings on Watercolour Paper, Smooth Drawing Cardboard, Ammonia-Free : Arts, Crafts & Sewing
How to use Masking Fluid for Watercolor Painting - Watercolor Affair