I use HOT water with vinegar, it seems to disolve the paste pretty well and its cheap. Patch big holes with drywall, if your old medicine chest was above the sink, you'll probably hang a mirror anyways, so your plasater jub doesn't have to be perfect! You should put a sealer over bare drywall, but a couple coats of paint will work. Bathrooms are my least favorite to do..too much stuff to work around and tight spaces. Good luck, have fun.