I would give the walls a coat of Pva 3-1 minimum as it shouldn't soak through the paint but sit on top,stabilising and forming a barrier.when Pva is set give another coat not as strong and skim.maybe try a small wall and leave it a day or 2.
Hot water or cement never made any difference for me,I use dirty water and if u ain't got any throw a couple of handfuls of plaster into a small amount water well in advance of doing the job and add to your mix.
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