do not, never ever, pva walls prior to tiling. tank the shower area and use a proper primer on the rest of the room . particularly new plaster. pva is a poor substitute for using the correct stuff. you only want to do it once, and right dont you.? go to trades direct on the web. you can get a tanking kit ,which will fully waterproof your shower area ( its very simple to apply), and bal apd for the rest of the walls delivered to your door. p.s. i am in no way connected to t d s.! LOL