I have removed the metal tiles which covered the lower half of the bathroom walls. What is left is the glue/mud that held the tiles in place. I have been scraping the excess off (in some spots is very easy to do since it is so old). To really get a smooth surface I know sanding is going to be needed to prep the walls for wallpaper. Is there anything easier that I could be doing? Some product that will remove the glue/mud? When sanding becomes necessary, what type of sander and grade of sanding paper would you suggest?