BobVila.com

You can leave the wallpaper up and prime over it as long as it is tight and not loose in any way. The reason they say to skim over the GARDZ is that is also used for priming walls after you have removed wallpaper and the walls were not primed and you tear the face paper of the drywall off. The GARDZ may very well work but I would prime with an oil based primer to make sure the wallpaper is locked down and moisture from your paint does not reach it. As far as skimming out a wall that is best left to a proffesional and you would only need to do that if the paper has a noticeable texture.