You will have to give them a bit of a rub down to 'kill' the gloss...then a coat of zinsseer bin primer, or something similar..
Waterbased masonry paint will work if you have any spare as its full of acrylic resin or acrlic primer/undercoat.
If you give them a good rub down and a coat of something like zinsser bin or dulux supergrip beofre the water based paint then they should be fine...you can even give them a coat of traditional oil based undercoat and then go over that with your water based paint if you dont want to go to the...
I have never painted them...but as rob says a couple of coats wont hurt..or even just the one..but make sure you rub it down after as there might be a few rough ibts that you might snag your kaks on
Its all down to the individual brands of paint...I have thinned dulux supermatt by more than 50% and its still done the job..home brand paints arnt as thick so maybe 20-30% would work
I think its down to whatever the job warrants..
If they need plastering and you can plaster then plaster them...if not why go to all that trouble if lining paper will do
Stripping the stain off will probably give you the best finish..failing that id go for a good rub down,knotting the knots, a coat of zinseer bin and then your top coats.