Primer instructions are included with every type of wallpaper sold. The instructions range from saying primer is not needed to detailed steps on how to apply it. Should you encounter problems with your wallpaper later, companies will usually ask first about the type of primer you used before applying the wallpaper. If you did not follow the instructions, you will probably be out of luck.
What It Does
The purpose of primer is relatively simple – it makes it easier for the wallpaper to adhere to the wall. This is particularly important for rough or particularly glossy surfaces. If a wall has been painted, a primer can make it so the wallpaper is less likely to roll or have issues. Primer also serves to hide color. When you stand back and look at your beautiful new wallpaper, you probably do not want to see a dark purple paint as part of the undertone.
Primers make it easier for you to remove the wallpaper. Since you are pretty much focused on applying (not removing) wallpaper, this may seem like an unnecessary concern. However, if you plan to own that home for the rest of your life, it is something you should definitely consider. Walls should not be layered with wallpaper, so consider what will happen in five or ten years when you want to give the room a new look.
Also, keep in mind that you may need to remove the wallpaper if the job was not done well. It could be a matter of days, not years, and you do not want to damage your walls in the process.
When You Need It
Primer is always a good idea, but you should need to follow the directions on the primer specified on the wallpaper roll. Even if your wall has a layer of latex paint on it, you need primer. Additionally, you will need primer if your wallpaper has pre-adhesive on it because the latex will soften under the paste. The end result is that the surface is not able to hold the wallpaper properly because it lacks a hard surface.
Primer prevents latex paints from softening under the paste. When the paste is applied, it sticks only to the primer, not the paint (or in the worst case scenario, the sheetrock behind the paint). If you ever apply wallpaper onto a latex paint, then you could end up tearing the sheetrock when you have to remove the paper.
Pre-coats are not the same thing as primer. They are primarily desired for a different kind of surface, such as glass or Formica – not painted walls. If your wall has a common latex paint, then you will likely need a primer.