Our 200 year old brick built cottage has a damp problem in the bedroom. It has had damp patches for 20 years to our knowledge. Having stripped off the current paper there is a combination of lime plaster and modern gypsum plaster where repairs have been made in more recent times. The damp is mainly in areas where the two plasters meet and the gypsum is damp yo the touch in spite of being at least 20 years old. Will we need to remove all the gypsum plaster and replaster with lime? Many thanks.