Can u skim on to plasterboard that has just had wallpaper taken Off it or do you have to pva it or put a bonding coat on it to stop it going off quick

I am assuming that the plasterboard has a skim coat on it as you have not said. If that's the case,then yes a 4:1 mix water/pva should do it to control suction skim when tacky. One coat of the pva should be enough as the wallpaper paste that's on the wall at the moment will have sealed the plaster a bit as well,but if you want to play it safe,then apply 2 coats let first one dry then second then plaster onto when tacky. Carl.

Exactly as Carl has said, pva prior to plastering, let the pva go tacky.