Masking Fluid Calculator — Watercolour Resist

A brush lays down roughly 0.005 ml per cm² per coat (a ~50-micron wet film). For a quarter-imperial painting (38 x 28 cm = 1,064 cm²) with 20% masking, that is about 1.1 ml per coat, or about 1.4 ml once you add the 30% allowance for brush loading and touch-ups. A 75 ml bottle therefore lasts many paintings.

Use old brushes or colour shapers (silicone tipped tools). Coat brushes with soap before dipping in masking fluid. Work quickly and wash immediately. Never use your good watercolour brushes — masking fluid ruins them.

Remove masking fluid only after the paint is completely dry. Rub gently with a clean finger or eraser. Do not leave masking fluid on paper for more than 24-48 hours — it becomes harder to remove and may damage the paper surface.

Yes, but test first on scrap paper. Some papers and paints handle this better than others. Staining pigments may lift slightly. Apply the masking fluid gently to avoid disturbing the paint layer.

Masking fluid always creates a hard edge. Soften edges after removal by wetting the area and lifting colour with a damp brush. Some artists prefer this crisp edge for highlights; others soften selectively.