Be sure they are absolutely dry first of all.
Any emulsion, I have been using the cheap own brand stuff from both B&Q and
Focus recently, with perfect results. Just water down your first coat and
then two thin coats would give a better finish.
Or Crown Covermatt or Macphersons' Eclipse, designed for use on new
plaster, allows any residual damp to escape.
Consider sourcing from one of the trade (Crown/Dulux) decorator
centres rather than B&Q etc.
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