How do you paint over wallpaper finished drywall?
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If you can't remove the wallpaper (and I highly recommend that you do), then I would prime the wallpaper with a good primer, let dry and then sand until smooth, Then paint.
Are you painting over a drywall skim coat over wallpaper or just over wallpaper? To be safe use oil based primer.
Technically you don't, you remove the wallpaper first. Score and steam has been my preferred method.
https://www.lowes.com/projects/paint-stain-and-wallpaper/remove-old-wallpaper/project
Any tool rental will have what you need.