I'd like to plasterboard out the old lounge doorway. I'll put sticky mesh tape across the join between plasterboard and old 30's plaster. Will this ensure I won't get a hairline crack between them in the future?
It will help prevent them, but you have two differing types of plaster, plus one is old and can start to fail at any time.
The movement between the two components, will at some stage create cracks.
Might be worth using a bit of Brick Reinforcement Coil
The best thing to do (if you have the depth tolerance) is to hack a margin of plaster off each side of the join, i.e. 100mm or so and allow the plasterboard to lap onto the masonry by said margin.
If depth is a problem use 9.5mm boards. Or skim the whole wall for a pro' finish.
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