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)
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Art masking fluid
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J.M.W. Turner, Chateau de Rinkenberg, on the Lac de Brienz, Switzerland,†1809, watercolor.
Removable blue masking fluid for watercolors. Special tip for fine details. Masking fluid makes specific fields water resistant. Dry thoroughly before
Schmincke Colored Watercolor Masking Fluid- 25 mililiter
Masking Fluid: Your Watercolor Secret Weapon!
Comparing Masking Fluids - Jackson's Art Blog
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Using the different art masking fluids
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