My husband hung the green waterproof type drywall in the new bathroom. After finishing the seams, I painted it twice with Kilz just for added protection from humidity and moisture of the bath. I painted it with 2 coats of the semi-gloss paint. After several days he wiped it trying to get off some thin set from the tile job, and the paint came off. You can actually pull it off like a rubber mask. What happened?