Plastering problem

[Lobster]

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

[Andrew Gabriel]

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.

[The3rd Earl Of Derby]

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.

[Lobster]

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

[Phil L]

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.