Having read a couple of notes regarding walls, we have a similar problem. Previous occupant, smothered the walls with very rough 'plaster' and now damp is seeping through leaving black mould. Do we remove 'plaster' and leave natural, or do we remove plaster, insert a frame, and cover with plaster board? If we leave it natural is there a finishing liquid we can use, to stop the mortar falling out - this is happening in one of the bedrooms.