use gypond, you cant go wrong , as for pva then something like unibond pva you would mix the first coat 4 parts water and 1 part pva and then give it a second coat at 3 parts water 1 part pva, where a lot of people go wrong it they will let the pva go off before laying on the plaster, what you want is the pva tacky to the touch then lay the palster on. never had any problems with it doing it this way in over 20 years