Wallpaper sizing creates a slippery, glossy surface on the wall, allowing easier movement of the wallpaper and easier application. Sizing is optional, but it can be very beneficial, especially for installing wallpaper over walls with a matte painted surface that does not allow for easy positioning.
Sizing can help the wallpaper grip the wall better and make it easier to peel off later if it needs to be removed. Walls with glossy paint already have a smooth surface, but sizing can make them even more receptive to wallpaper. Glossy-painted walls should be lightly sanded to help the sizing stick better. Sizing is applied with a paint roller. Wallpaper and wallpaper sizing should be applied only to walls that have been painted or primed and never to bare drywall.