What do you put on new plaster before tiling?
Subsequently, one may also ask, do you need to plaster before tiling?
Tiling on plaster is ok, as long as your not fitting large format tiles. Still prime the plaster (obviously when dry) with acrylic primer. NOT PVA. Why plaster though..not really necessary.
Secondly, how do you seal a wall before tiling?
PVA can be used to seal any type of porous surface,including plasterboard and if it is done properly it will give you a much much better job than working on unsealed walls. To seal a wall the PVA should be dilted at about 20 - 25% PVA to 80 - 75% water.
Ideally leave new plaster (bonding plus finish) 4 weeks to dry before tiling. Dot and dab plasterboard with plaster skim can be tiled in 24/36 hours. If you don't skim the plasterboard you can tile it immediately but prime it first with an acrylic primer like BAL APD or SBR.