By small areas I mean not entire walls. I'm pulling my kitchen apart and when I pulled the tiles off some of the plaster came off (a couple of areas of about 1.5m squared each. Now the concrete below these is a little crumbly so someone at work told me to get some pva, dilute it and then paint it on first before putting the plaster on. I did this in a small area yesterday but am not convinced it's the best thing to do as the plaster I put on still shows damp patches presumably as the moisture is not soaking into the walls. Have I done this wrong? I don't want to cause more damage...
It will take more than 24 hours to fully dry. Our en-suite was re-plastered a few months ago and it took several days before it was all an even colour.
It has to stay wet until it's set. If it dries out first, it won't set. It only takes some hours to set though. Usually takes a day or so for just a reskim coat to dry, but a week for full undercoat and skim to dry. If the underlying wall is new too and still damp, then that will increase further. The weather and ventilation also make a big difference.
What's usually recommended is dilute 1 part PVA 5 parts water and paint on copiously, and leave to dry (hopefully it will mostly soak in first). This glues the surface particles together so it's not dusty. Follow this with dilute 1 part PVA 3 parts water, and plaster onto it while it's still tacky. That helps to glue the plaster to the surface.
In practice, exactly how you do this isn't particularly critical. Some plaster (e.g. bonding undercoat) has a glue in it anyway and the second coat of PVA isn't needed (just paint water onto the wall first if it's still particularly absorbant). Finish coat plaster can have more trouble sticking to some surfaces (including set plaster), and the PVA as a glue helps here. (You can plaster a sheet of glass this way.) When plastering, you aim to put finish coat on something like 2-4 hours after the previous plaster coat (or 24-48 hours after a sand and cement coat), in which case you don't use PVA. If you leave it much longer, it's best to let the underlaying coat dry and then apply as for a reskim.
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