Watercolor resist is basically just a tiny bottle of tinted liquid latex. There are a number of ways to mask out areas of watercolor and ink drawings, but I like the way the resist leaves natural brush lines around and inside the masked area.
You'll need: a bottle of "Art Masking Fluid" (I used Windsor and Newton, above), and some cheap, relatively stiff, paintbrushes. Paint the resist onto the page, in a medium thick layer, the thinner the coating, the more difficult it is to pull it up and peel it off. This latex is tinted so that you can see where you've painted.
When you are done painting, immediately dunk the brush in warm water. If the latex is allowed to dry on your brush for even a moment it will turn the bristles into a horrible, gummy, mess.
Allow the resist to dry completely, then paint over it with watercolor, ink, or gouache. Once the paint is dry, you can carefully peel up the resist. The results are kind of magical.
|This is the fun part|
You can use the resist to cover an already painted area, as long as the paper is fully dry. Any dampness in the paper will cause the resist to fuse with the paper and become very difficult to remove.
Above, you can see how you get a nice continuity to your brushstrokes when you're using the masking fluid. It would be useful when blocking out characters against a cloudy sky, or to create clean areas next to any kind of splatter texture. I searched around online to see who had made amazing use of this technique, and found a few interesting results, but I bet you guys can do better. Send along any images and I will post them later!