Our guttering had blocked, and was causing water to flow down an exterior wall - unnoticed by us, until we found damp patches on the interior wall.
We got the blockage removed and the guttering is working normally. I stripped off the wallpaper on the interior wall to expose the plaster and try to dry it out. There was some blackspot mould on the plaster. I've cleaned it with bleach and twice with a fungicide spray. The black mould has disappeared and the wall is no longer damp to the touch. But a white fluffy mould keeps appearing on the plaster, apparently at the edge of where the damp patches were. I have removed this mould and resprayed the patches where it appears with the fungicide spray, but it keeps coming back.
What do I do? Do I have to just wait for the wall to dry out? How can I stop the white mould from coming back?
Brush off the mineral salts as they appear and let the wall dry out until no more salts form on the surface. Ventilate the room as much as you can and leave unpainted until at least mid-summer.
I would not recommend you paint the surface, this will seal in the moisture and you will end up with mineral nodules coming through the painted surface.
You should have no problem painting over the efflorescence after brushing off, if the surface is dry this will remove any sulphates that have been brought to the surface if you want to make sure that no damage will occur to the paint finish use a contract matt emulsion these are formulated to allow the passage of small amounts of moisture from under the surface of the paint, the efflorescent may still return but will not damage the paint however if you have damp mark there constantly these are nitrates and chlorides and are hygroscopic they absorb moisture from the atmosphere, they are a different matter they do need to be sealed with an alkali resisting primer as rosebery has suggested to stop them from absorbing moisture.