I rented a house out to my son and allowed him to use the walls of one room as his canvas for artwork. Now he's moved and I have to cover his unconventional murals. I'm planning on using Paper Illusions on these plaster and lath walls but the artists crayons he used are quite waxy. The house needs to go up for sale soon and I don't want to spend days stripping his artwork. I'm not sure the wallpaper will stick to it as is. Any ideas on how to remedy this? Thanks!