Eleven years ago I cleverly glued a cork border at chair-rail height all around my son's bedroom wall. It served its purpose, but now he is married and I want to repaint his room. I have scraped off all of the cork, but the glue spots are left on the wall. they will not scrape off. I have sanded them flat in some spots, but this also heats and spreads out the glue residue. I don't remember the type of glue that I used, but I remember that it was a multi-purpose, gel type of glue. Can you give me some guidance on the best way to prep the wall before painting?