Try "Mold Builder Liquid Latex". it is sold at Hobby Lobby and other craft stores. It is pretty thick stuff and you may need to thin it slightly with water. If you keep the lid on tight it will not thicken or set up. It is far superior to wax in that when you peel it off your piece, there is no residue that repels glaze. If you make a mistake, just rub it off with your finger when it cures out. Since you peel all of it off after the glaze drys, you never get isolated blobs of glaze that stick to the middle of resist lines.
The container it comes in is kind of big so I pour a couple of ounces into a old pill bottle with a good tight lid, and brush from that.
I had a clay body that the latex was hard to see on, so I added a couple of drops of red food color. worked fine. Just don't add any color that is not an organic pigment that will burn off in the kiln.
There has to be a down side to any story...right? The down side is that if you leave the brush sitting around, the latex will set up and you will lose the brush. No amount of scrubbing, solvents, boiling, cussing. will bring it back. The work around is simple. Every once in a while clean it in cold water. Swish it around in a may jar full of water. My sink is close so I jut hold it under the tap.