i work as a craft mason so old buildings are my speciality, if i were you i would try an alkaline sealer, you wont find this in homebase or b and q , you will have to go to a dulux trade centre or similar trade shop
, it is most probobly due to effloresence ( a build up of soluble salts ).
just a quick note. if your building is solid stone, then despite what many people might tell you it is unadvisable to tank the wall. beleve me i know what i am talking about, i have to remove tanking a lot to sort out damp problems !
ideally in a perfect world it should be rendered or pointed with a lime mortar on the exterior, and depending upon the finish inside it should also be rendered smooth with lime , and then skimmed with a lime putty ( this when applied properly can produce a marble like finish !! ) . im sure you have heard people talking about letting the walls breath, it is important if you dont have a cavity construction.
allthough im ranting on and realise you are just trying to paint a room , i thought id let you know because tanking can be a very expensive mistake ( yes a lot of the time it is okay , but problems generally get pushed in to other parts of the building). i get a lot of work sorting out damp problems in old buildings.
and for the right job i will travel anywhere in the world to practise my craft
!!! KERNOW BYS VYKEN !!!