In article ,
"JustMe" writes:
Hi folks,

My house is exactly 100 years old. About 3 years ago most of the plasterwork
was skimmed. The original plaster was left in place and a new, smooth coat
was applied on top. I moved in 2 years ago.

The problem is that the old plaster foundation on which the skim was applied
doesn't appear to be sound in some places. In some areas the base isn't
stuck to the bricks so when I to apply pressure with my hand, I can feel it
moving up to and away from the brick by a few mm. Where this is very bad I'm
having to arrange for all plaster to be hacked back to brickwork and new
plaster applied, because the plaster is cracking all over,

As you suggest, the plaster wasn't in a fit state for reskimming.
You don't have to fix every last bit where keying is lost with the
wall behind, but a large area of movement like this does need
fixing, and it's very easy to do just before you reskim the whole
wall. It could be dangerous to a small child or elderly person
if it all fell out on top of them.

but there's one
area where there's just one crack, running from the top corner of a door
frame diagonally up to the ceiling. I previously repaired it by scoring it
out, filling it and repainting, but after a year the crack has returned.

I really don't want to hack this entire wall back for one crack - can anyone
advise a solution that'll last?

That's a different situation. 100 year old house will be built
using lime mortar which allows the house to routinely move
around. The original lime plaster similarly moves, and the horse
hair in it prevents it forming large cracks. I assume the reskim
was done with gypsom (pink or grey) plaster, which isn't flexible,
so when there's movement in the wall behind, the gypsom plaster
will crack. Often this doesn't happen enough to worry about, but
if you have a crack where it does, then you could try raking it
out and filling with a more flexible filler such as decorator's
chalking. This shrinks as it sets, so you might need to go over
it 2-3 times to get a flat level finish. The more movement you
are getting in that crack, the wider strip you will need to rake
out and replace with flexible filler to accomodate the movement.
(I assume we're talking about hairline cracks here, rather than
something you can push a pencil in, which would be a different
problem.)

The other option would be to replaster with lime plaster, but
that probably won't work with just a reskim -- you would need to
replace the scratch coat too.

--
Andrew Gabriel