Dark Mark's when mist coating new plaster

Hi. So the whole surface was a light pink colour after drying, and this was a plaster re-skim over existing plsster? If my questions are correct then..... It sounds like an area of plaster that has been over polished in the final trowelling stages, common occurence. Sand it back slightly with 120 grit yellow or green the lightly fill over that with a coat of filler (easifill or toupret the go-2) then sand that level when set and re-mist coat that area

Hello there did you notice these areas to be slightly shiny before you applied a mist coat? If so this is a result of over polishing (trailing) during the final stages, this is no big deal just simply rub down gently apply filler Only enough to cover the surface of the patch as you will have to rub this down when dry, when rubbing down be gentle as It won’t take much to remove filler, ensure all surfaces are level and then apply another mist coat.

It sounds like there’s a deep damp spot. Try and get some gentle heat on it. Hairdryer or similar.

Hello there, There are several possibilities. Assuming the plaster was only a thin skim coat and there are no deep plaster areas still drying out and the rooms are well ventilated. If you've plastered over existing walls then there may have been something which has contaminated the wall and is bleeding through, i.e.an oily mark, stain from a previous leak or something metallic like a broken screw or nail stuck on the wall under the plaster. Failing that, it could be a damp patch as a result of something outside i.e. broken rainwater goods, missing roof slate/tile, or rising damp from the ground. If the dark patch is dry then try painting over the affected area with an oil based undercoat, leave to dry overnight and then emulsion over. If the dark area is still wet then further investigation as to the cause is called for by a reputable builder. Hope this helps.