aharbour

Can anybody help me regarding Saltpeter? We have just started procedings to purchase an house in the Dordogne. The house hasn't been lived in for sometime and the what seems similar to plaster is crumbling on some walls, the Immobilier tells me this is common due to the Saltpeter being drawn out of the stone and suggest bording the walls, but this will remove the natural character and not something we really want to do. Can anybody suggest a possible solution? Is there a product or a process the French traditionally use to cover their walls? I am pretty sure the current covering isn't plaster like we use in the UK. Any help would be appreciated.