Waterproof Sealer for Interior Walls

Hi Guys, Hoping you can help. First time home buyer who has decided to do a lot of the work myself in doing the place up and ive got a waterproofing question I could use some assistance on. Basically... ive had my bathroom completely replastered (over plaster board) so I have nice new surfaces to work with. I am going to seal over the new plaster with a mix of white emulsion and water (80% to 20%). I had picked my main paint to go as the top coat (light grey emulsion). I thought what I bought was waterproof... turns out that it isnt. So what I am looking for is a cost effective way of waterproofing that paint, instead of having to shell out for more paint. Just something that I can paint over the top of my final coat (once dry) to seal the walls. Ive looked around quite a bit and most seem to be for external walls... or sealing masonry and im not really expert to know if that would do the job? Ideally this needs to be clear, matt based finished and water based. But at this point im open to any ideas. Any advice or assistance would be very much appreciated. Thanks Jason