You've received a lot of very poor advice here, I'm afraid. Old houses should never be plastered with gypsum. It's like putting a coat of plastic over the walls. You're going.to end up with condensation and mold issues, and possibly some of the plaster failing.
Furthermore it's not the wisest to use NHL like for plaster in such applications. You want a proper Calcium Hydroxide plaster, hydrated lime. There are specialists who sell it in bags or large sacs. NHL doesn't have the same level of transpiration as hydrated lime.
Probably the best solution in terms of fighting condensation and salt issues, as well as providing insulation, is to use a hemp lime.