Salt masking is when you apply wet salt to the surface of the
model in random patches. After it drys and your airbrushing it will
blow some of the salt off and some stay in place. When the paint drys
just rub the salt off and it leaves behind rough edges and chipped
paint look.
Liquid friskit is a latex masking agent used in art work.
You can find it at most art supply and craft stores. You just brush it
on, wait for it to dry, and after the paint drys it peals off the
surface. The nice thing is that it wont peal up the paint being masked
like tape can do at times and it doesn't leave behind any tacky film
when its removed no mater how long it sets on the surface.