Masking fluid allows you to create complex negative spaces with watercolors or gouache. Using the tip of the masking fluid, draw with the masking fluid to define areas you want to keep blank. Allow it to dry, then paint over the masking fluid. The paint won't adhere to the areas you've masked, and when the paint dries, you can simply rub the masking fluid off with your finger, revealing beautifully crisp white space underneath.

The plastic applicator tip on this masking fluid is perfect for sweeping lines and fine details.  Avoid tearing your paper by allowing the masking fluid and watercolor to fully dry.