Sealing Plasterboard

You can use normal vinyl mat emulsion straight on plasterboard.It is fine. Alternatively you can use ( if you got any on hand) superlatex emulsion as the first coat. Good luck

Not necessarily seal, as the above answer suggests, but a little water in the emulsion for the first coat or two will make it easier to apply as the plasterboard will soak the moisture up quite quickly regardless. No need to use PVA or wallpaper paste or anything like that. Also as a side note, if you have tape joints in the boards, i.e. medium / large areas that have been filled over with board finish or fine filler, it is always best to treat these areas like you would treat new plaster and seal them with a watered-down mist-coat of emulsion first as they are highly absorbent, otherwise you may find the paint peels off during subsequent coats.

Hi. You can use any emulsion of your choice. But water it down a little as you suggest. This isn't for the purpose of sealing it, but the paper will quickly absorb the paint esp on the first few coats. So easing it with some water will reduce the possibility of having brush and roller marks

3 very good answers above and would have said the same so wont try and repeat what has already been said. I would suggest sealing your perimeter edge with a translucent silicone at the end to prevent any moisture getting through. All the best