These 2 questions come up oftenÖ..
1. Definitely no you can not
& should not
plaster over Wallpeper
2. Yes you can plaster over sound well stuck paint paint (test it first!)
the use of a thinned coat of PVA adhesive is the normal process - leave to dry then apply a thicker coat and when its tacky apply plaster.
3. As a decorator i mostly use Toupret Joint & Skim ready mixed
Toupret Skim Flex can be applied to:
Tiles, ceramics and glass tiles
Polystyrene and cork
Plaster, gypsum plaster and plasterboard
Old painted surfaces
Any conventional surface preparation filler
Concrete and aerated concrete
Cement, breeze blocks, brick and stone.
but not! Wallpaper
& its ready mixed!
always test that what your applying to is clean, sound