Painting Over Wall Blemishes
In many cases the most successful and least time-consuming approach is to either remove and replace the drywall (and catch up with any issues exposed when the wall is open) or to lay a fresh layer of drywall on top of the wall that had wallpaper on it.

IMHO the skim coat of mud (sand optional) will do the job without any "special sealer" first, but it's a time consuming approach .vs. new drywall.