scrape really good. roughen everywhere!!!! fill all holes and big
cracks with drywall mud, it drys slower than patching plaster and is
easier to use
Then prime with bin or kilz primer sealer, that will help adhesion...
wait a few days then skim coat everything with drywall mud, use a brush
to give it a slightly textured finish that will hide imperfections:)
Try a single wall first and try it! It worked wonders here, before we
bought this house the walls were damaged and never fixed properly from
a bathtub intentinally left running, they were being evicted.
you would never know this occured!