I use masking fluid, the Windsor & Jensen stuff from Michaels. Found it in the art section, with the artists paints, not the crafting section. I put it on with an old brush and after it dries I put on another layer, then another and so on. I like to put it on thick, that way it comes off easy. I also use a sharp razor to cut around the edges to ‘break the seal’ sorta and a SMALL sharp pair of jewelers tweezers(or even pointy eye brow tweezers) to gently lift and pull it off. I haven’t had any trouble at all, besides the paint pulling up if I got the masking fluid on the sculpt and then painted over it.
I always use it, especially if I’m using pastels because I use a spray fixative that would coat the eyes, horn or gem of said critter….