As I do every so often, I've just had another go at doing some
plastering, just to confirm to myself that I *still* can't do it
properly and should get a pro in....

Didn't go all that badly actually and I'm quite pleased with the results
(base coat only - I'm not trying the skim coat!) but for one problem -
as it dried out, quite large cracks appeared across trhe surface and
between the wet plaster and the adjoining old dry plaster. What am I
doing wrong?

Details:

Substrate was a mixture of bare (old) brickwork and crappy old dusty
plaster which I was patch-repairing. Applied dilute PVA (1:4) just
before plaster.

Plaster was bonding coat (brown, coarse, gypsum); was fairly sloppy mix
but stiff enough to stick to the walls no problem; was a warm day.

Thanks!
David

In article ,
Lobster writes:
As I do every so often, I've just had another go at doing some
plastering, just to confirm to myself that I *still* can't do it
properly and should get a pro in....

Didn't go all that badly actually and I'm quite pleased with the results
(base coat only - I'm not trying the skim coat!) but for one problem -
as it dried out, quite large cracks appeared across trhe surface and
between the wet plaster and the adjoining old dry plaster. What am I
doing wrong?

Details:

Substrate was a mixture of bare (old) brickwork and crappy old dusty
plaster which I was patch-repairing. Applied dilute PVA (1:4) just
before plaster.

Plaster was bonding coat (brown, coarse, gypsum); was fairly sloppy mix
but stiff enough to stick to the walls no problem; was a warm day.

You didn't necessarily do anything wrong, in that cracking of
the bonding coat can happen, and providing it remains bonded,
it's not harmful. If the background is very absorbent, e.g.
plaster edges, thermal blocks, some indoor bricks, etc, then
two coats of PVA is better -- the first at 1:5 and allowed to
soak in and dry, and the second at 1:3 applied and plastered
over whilst still slightly tacky.

You might have had the bonding coat too stiff -- a good test
is that is should stay on the hawk if you hold it vertically,
but only just, and a slight jolt of the wrist should cause it
to slump off (verses finish coat which should slide straight
off when tipped vertically).

Another possibility is you did too much thickness for one
coat in a single go. Plaster shrinks as it sets, and although
much of this is taken up with the coat just getting a bit
thinner, in the case of a thick coat, some cross cracking is
probably inevitable. (This is much more important in the case
of finish coat, which is why it must be done in very thin
coats.)

Also, the temperature is rather hot for plastering at the
moment -- it sets much faster so you need to work faster
than you otherwise might.

Just look in the cracks as extra keying for the finish coat,
although you'll need to PVA the scratch coat if you aren't
applying the finish coat immediately.

--
Andrew Gabriel

Lobster wrote:
As I do every so often, I've just had another go at doing some
plastering, just to confirm to myself that I *still* can't do it
properly and should get a pro in....

Didn't go all that badly actually and I'm quite pleased with the
results (base coat only - I'm not trying the skim coat!) but for one
problem - as it dried out, quite large cracks appeared across trhe
surface and between the wet plaster and the adjoining old dry
plaster. What am I doing wrong?

Details:

Substrate was a mixture of bare (old) brickwork and crappy old dusty
plaster which I was patch-repairing. Applied dilute PVA (1:4) just
before plaster.

Plaster was bonding coat (brown, coarse, gypsum); was fairly sloppy
mix but stiff enough to stick to the walls no problem; was a warm day.

Thanks!
David

Could be anything from too warm the room,dry old plaster,or too thin a mix
on a warm wall,plaster being off?

I have found if I'm to plaster on old plaster where its dusty or crumbly,is
too use scrim on the wall even if its an entire wall or a patch,this
reduces the risk of the plaster cracking and if it does crack then I know
it was another problem that made it crack.

--
Sir Benjamin Middlethwaite

Andrew Gabriel wrote:
In article ,
Lobster writes:

Didn't go all that badly actually and I'm quite pleased with the results
(base coat only - I'm not trying the skim coat!) but for one problem -
as it dried out, quite large cracks appeared across trhe surface and
between the wet plaster and the adjoining old dry plaster. What am I
doing wrong?

You might have had the bonding coat too stiff -- a good test
is that is should stay on the hawk if you hold it vertically,
but only just, and a slight jolt of the wrist should cause it
to slump off (verses finish coat which should slide straight
off when tipped vertically).

Thanks Andrew. In fact judging by the above, my plaster was too sloppy;
it would certainly have slid straight off sometime before the hawk was
vertical. A useful test which I'll try next time! I was anxious not to
make the mix too stiff in view of the warm weather; worried about it
going off too soon.

So, conversely, could using too wet a mix make the plaster crack? I
must say, it seems to have stuck well to the wall, so I'm not unduly
bothered by it.

David

Lobster wrote:
As I do every so often, I've just had another go at doing some
plastering, just to confirm to myself that I *still* can't do it
properly and should get a pro in....

Didn't go all that badly actually and I'm quite pleased with the
results (base coat only - I'm not trying the skim coat!) but for one
problem - as it dried out, quite large cracks appeared across trhe
surface and between the wet plaster and the adjoining old dry
plaster. What am I doing wrong?

Details:

Substrate was a mixture of bare (old) brickwork and crappy old dusty
plaster which I was patch-repairing. Applied dilute PVA (1:4) just
before plaster.

Plaster was bonding coat (brown, coarse, gypsum); was fairly sloppy
mix but stiff enough to stick to the walls no problem; was a warm day.

Thanks!
David

not enough H2O, both on the face of the brickwork, and on the underside of
existing plaster....they need to be drenched in this weather, saying that,
bonding/browning will shrink under most circumstances, but a good soaking
with water prior to applying PVA will help minimize this...remember, only
rough in (apply backing coat) what you can skim in the same day
(bonding/browning only - render doesn't matter)...browning is practically
impossible to skim (and get a decent finish) once it's completely dry and no
amount of water/pva will stop the suction.