DIYbanter - View Single Post - Internal rendering/Plastering with Sand/Cement/Lime Mix

In article ,
(Anna Kettle) writes:

My immediate reaction to the 'cracks are worse ...' is that you have
not killed the suction enough where dry materials meet the mortar. But
then you say no. Could you try wetting the wall (and especially the
edges) well then allow time for that lot of water to soak in before
wetting again - say three sprayings in 20 minutes? Too wet a wall does
not give that problem, the plaster just won't stick to it.

Be careful with thermal blocks. They expand when they get wet, and
contract when they dry out (blowing the plaster and possibly cracking
the blocks), so don't soak them, just wet the surface. Use PVA to
limit the suction by sealing the surface in that case rather than
lots of water.

Another cause for large cracks can be applying too thickly.
Plaster shrinks as it sets. A thin coat shrinks by getting
thinner which doesn't notice, but a thick coat will shrink in
all directions, resulting in cracking.

Hairline cracks in gypsom plaster used in a lime mortar building
are pretty inevitable. Such buildings routinely move/flex, and
gypsom plaster doesn't. You just have to put up with it or switch
to lime plaster.

--
Andrew Gabriel