Plastering over wallpaper is not a viable option. It will cause disaster after a few days or weeks because the plaster will crack and fall away. You will have to scrape the wallpaper off for the plaster to get a good key to the wall behind.
The wallpaper will become wet from the plaster and suck out the moisture, once the moisture comes out of the plaster it will crack and fall away.
The Wallpaper Will Bubble!
Another serious issue is bubbling, yes you can key the wallpaper with a knife and it might connect for a while, but it will eventually fail and you will have to scrape all the plaster off and start again!
- The wallpaper will bubble
- Plaster cannot key to wallpaper
- The wallpaper creates a barrier between plaster and wall!
Plaster needs to dry slowly and a good surface to grip to. The wallpaper will become a barrier no matter how much PVA you spread over the paper. For the wall to look presentable you will have to put in the preparation to make the job good!