This is what happens when an impervious paint is used over a limestone lime mortar surface.
The mortar dries out and turns to dust and the whole surface cracks and delaminates. Eventually the wall must be demolished.
PCC have presided over a mass destruction of such buildings by insisting that a cement mortar render covered by impervious paint finish be used.
TODAY they are allowing cement render of lime mortar walls on listed buildings and in Conservation Areas.