When removed from a wall, wallpaper paste can leave behind a glue or adhesive that does not interact well with paint. To avoid damage, it should be removed with a scraping blade, sandpaper or a chemical remover.
Wallpaper is affixed to a wall with paste or glue. As time goes by, the chemicals in these products can seep into the wall and grow hard. When the wallpaper is removed, residue is left behind, and it is inadvisable to simply paint over it. Scraping the wallpaper away can leave gaps in the wall that can be repaired with plaster filler or sandpaper so that it is safe to paint again. A chemical remover also works, but it can leave the walls damp. The surface should be wiped down before painting, and latex primer allows for optimum paint application.