I've removed a large bathroom mirror that had been attached to the wall with adhesive or mastic and had a frame screwed to the wall around the perimeter. The adhesive is fairly dry, but doesn't budge under a scraper. I'd like to clean the wall and apply fresh paint. How do I remove the adhesive without gouging the plaster's top coat? How should the plaster be prepared to allow paint to adhere and not stain? Any input is welcome!