Answer from the Painting Guide: Masking fluid should dry fairly quickly, so it doesn't lift off when you run a brush with paint over it. But it doesn't dry like paper; it retains a rubbery, soft feel to it that can feel tacky if it's applied very thickly or it's an old bottle that's started to get a bit thick and separated out.
Answer from Jon Rader Jarvis: A frisket is any material that protects areas of a work from unintended change. Sometimes a sheet of adhesive backed paper is called a frisket sheet. The liquid in bottle form is called a liquid frisket.
The age and length of time on the shelf can affect drying time. Most masking liquids use ammonia to keep a rubber masking liquid (latex) in solution. As the solution dries it remains tacky so that it can be removed by sticking to a kneaded eraser or an "ugly" (dried rubber cement ball) used to pick up sticky stuff like Frisket. Sometimes a finger tip is enough to remove it. It's designed for water soluble media: watercolor, gouache, casein, tempera. Thin acrylic can work with it but buttery acrylic is too thick, making removal difficult or impossible.
I'm not a fan of masking liquids. For me they are too harsh tearing soft watercolor paper and leave hard edge traces or dark bits that change the paper surface for future over-painting. I use Scotch tape or bees wax candles for resist purposes, but that is a personal bias.
Jon's answer first appeared on the About.com Painting Forum
