I am filling in an area of old brickwork which was exposed when I removed some wall tiles. The hardwall bag specifically states not to use PVA which is fair enough for the brickwork which I will damp down first but what about the edges of the existing lime plaster which I am levelling up to. The old lime plaster is very dusty and liable to disintegrate, so will pva be necessary on the edges or will water do? I will be having the walls skimmed afterwards.