In the watercolor technique, you have to learn how to reserve whites in advance with the masking liquid and then apply the paint around it.
Using Masking Fluid with Watercolors – Owl Ink
Working with masking fluid — Charlene Collins Freeman Art
For masking areas of work needing protection when colour is applied in broad washes. A pigmented liquid composed of rubber latex and pigment. Do not apply on damp or soft sized paper. Remove as soon as possible after application. Brushes can be cleaned if washed immediately with water after use.
Winsor & Newton - Art Masking Fluid - Yellow- 75ml
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Masking Fluid Guide And Reviews
Pebeo liquid frisket for watercolor and airbrush is applied to a surface to mask areas where color is not desired. The low odor formula contains synthetic latex and is AP non-toxic certified, while the original formulation contains natural latex and a CL irritant warning. Either one may be applied with a brush or applicator bottle. Apply to all shades of paper, allowing 10-15 minutes drying, before color is applied. To remove the drawing gum, gently rub it with a finger or eraser.
Pebeo Liquid Latex Masking Fluid Drawing Gum, 45ml/1.52oz Bottle