Walls – Touch up Plaster/White Cement

If the surface is undamaged and it sounds solid just leave it to dry out completely. If it's now dry to the touch then it is drying out. If there was a lot of water coming in it will take a while to dry out.

Once it has dried then seal it with a coat of thinned white emulsion before putting the main colour on or wallpapering.

One caveat though - it could be permanently stained so it might still look damp but be perfectly dry - so even if the discolouration hasn't changed in week (or so) it will be safe to decorate. What you could do in the first instance is just paint. If there is still a problem with the damp it will quickly show up and you can then look at replastering.

I'd use veneer plaster. Your instinct about joint compound and spackling is on target, and you might get Plaster of Paris to work if you get the mix perfect - but why bother.

My guess is that the people you've been talking to at hardware stores either don't have a clue what you are looking for or want to direct you to something in stock. I'd check at a building supply store (you might get lucky at a big box store, especially if they carry blue board) that contractors in your area use.

Veneer plaster is extremely uncommon in my area, and I can get it when I need it from any of the big chain stores (although I usually have to order it - they never stock it).