I recently bought an old house (200-300 years old). I have a large walk in cupboard which has crumbly plaster (the bottom 50-100 cm) with black mould on parts of it. (the problem wall is an outside wall with some shrubbery growing against it) I have scraped off all the loose plaster (it has a reddish colour to it), but have not gone back to the brickwork. What is left is powdery (if you rub it, it comes off a bit like sand). I have also scrubbed the area with mould killer to get rid of the black mould. Should I wash it with a PVA/water mix to reseal the powdery plaster? If I do that can I then put new plaster on top, if so what type?