I have been doing some paintings where I use rubber cement to mask out areas of the painting. It works fine when I am painting thin layers of paint over the top but it did not work well when using thick layers of paint. I am wondering if you know of a good masking medium for acrylic paints.
Thicker films are more difficult to mask for, especially if you let the films dry before trying to remove the masking. We have used simple masking tapes for up to 1/2" thick layers, but right after you apply the product, you need to remove the taping so it does not get trapped.

If you have a concern about paints seeping under the tape and causing a jagged edge, you can apply GAC 500 at the edge first, let it dry and it will keep a cleaner edge of the tape when you apply the actual paint color.