by Doc Rob
Masking fluid might help. Basically it's liquid, brush-on latex in a 2 oz. bottle.
The stuff I have is Susan Scheewe Blue Masking Fluid; you'll find in the Artists section of Michael's (where the loose artist brushes, paint trays, etc. are kept). Can't for the life of me recall who on the board recommended the stuff to me, but they swore by it; I've only used it once, with spray paint (which turned out to be a bad idea as it's only meant to handle light brush over, not complete coverage like I did) but it should work fine with acrylic paint--just brush on, let dry, paint, then rub the dried masking fluid off.