Masking fluid is latex (aka liquid rubber) plus additives that lengthen shelf life, thicken, and promote smooth flow. In addition, many masking fluids have a light, non-staining colorant added so the masked area will be visible on white paper.
Of course, masking fluid can be used to reserve your whites when creating a watercolor painting as in example 1. However, you can also use masking fluid on previously painted areas with some success such as in example 2. When the dried mask is removed, the color underneath is preserved.
And did you know that you can stamp with masking fluid? In example 3, you can see that the impression is imperfect due to the liquid nature of the masking fluid. However, I think it creates a cool, weathered-looking batik effect that would be great for backgrounds. Just be sure to wash all your stamps afterwards.
Masking fluid can sometimes be a little bit tricky to use so here's some tips:
- Whatever you do, don't use a good brush! Masking fluid is famous for ruining brushes so either wash your brush thoroughly and immediately after application or use something else to apply the mask such as the bottle itself or the handle of a paint brush.
- Don't shake the bottle of masking fluid or you'll squeeze out hundreds of bubbles all over your paper. Roll the bottle gently on a table to mix and pop any bubbles with a pin.
- For best results, let the mask dry overnight but not much longer. The longer the fluid is left on the paper, the harder it can be to remove.
- Old masking fluid can also be tougher to remove so if you are having trouble removing the mask, you might need a fresh bottle. Masking fluid has a reasonable shelf life but we all have those supplies that we buy to experiment with, set aside, forget about and then dig back out years later. Sometimes those supplies, such as masking fluid, deteriorate over time.
- Use a piece of rubber cement pickup to gently remove the mask. Work a small area at a time and clean the resulting balls of latex off your pickup as you work. Remembering using rubber cement as a kid? Masking fluid is the same sort of stuff.