What is the difference between priming and “sizing” and which one should I do?

Primer is a protectant/sealer that prevents the paste from being absorbed into a wall and allows it to grip to slick surfaces. It is highly recommended that you prime a wall before hanging wallpaper because it creates an even surface for the wallpaper to adhere to as well as makes the wallpaper removal process much easier should you ever decide to update the wallpaper. “Sizing” a wall is thin coat of paste applied to a wall to create additional grip and make it easier to slide each strip of paper onto the wall. “Sizing” is often recommended for porous, repaired, or plastered walls. However, many combination primer/sizers are available and can be used simultaneously for a wallpapering project.