Can't believe it's been seven years.Zombie thread resurrection alert!
Diluted PVA makes a good sealer on raw plaster. Not sure it would keep for seven years though.On the subject of lining paper I have tried that before but found it didn't looks as good as a painted plastered wall. Depends on the finish/prep of the wall of course which is the bit I want to do well!
I'll have a look at Polycell basecoat but in the past I've just used watered down emulsion as a wash/sealing coat and that has worked fine.