I have recently moved to a new property. On one inside wall there is an indication of salt (the white crystalline stuff). This I understand must be removed. On an adjacent wall there are black patches that seem to be within the plastering. The wall is dry and was wondering if there is need to remove the plaster or if it may have been caused by someone putting up wallpaper over damp plaster... I do believe that whoever did worl on the house previousy is a total rogue.