Hi guys,

Been putting a hardwall coat on a concrete block wall, covered in places with old (but rock solid) plaster.

After a couple of hours the new plaster has cracked. Is this because of the high suction of the background? I thought Hardwall was intended for these backgrounds. What's the best course of action here? Can I stick another layer of hardwall over the cracked surface before the final skim? I was originally planning on doing a couple of Hardwall coats anyway as it needs to be pretty thick in places. In future is it best to PVA the concrete/Thermalite blocks first?

Cheers

just silly high suction mate, long as its only hairline on the hardwall this is reasonably normal..
thermalites are a bugger for it..
i mentioned dampening down on the other thread, but dont over do it as someone else mentioned - it can crack the blocks themselves....

just skim it mate provided you aint left it too long or pva then skim, the cracking is due to shrinkage, this is what i meant by not skimming it too early, it would have pulled the skim in with it and cracked the skim as well...

now its done its shrinking youre fine to skim it...

provided the suction wasnt so high the cracks are big crevasses and the hardwall is loose on the substrate, just crack on mate

bg do a product known as gypprime, its specifically designed for priming silly porous surfaces such as thermalites prior to application of backing coats such as hardwall or toughcoat...

reason i didnt mention it before is because its a) expensive and b) comes in big tubs of which youd use about a tenth..

dont panic mate, it'll be fine...

_________________
http://www.peterborough-plasterer.co.uk/

Brilliant!

I'd forgotten you'd mentioned it on the other thread! Was panic-stricken about the bricklaying at that point and the ultra-sucky blocks!

Thankfully they're only hairline cracks and it feels pretty solid so I'll give it a skim and see how it goes.

Thanks again.

Thanks to the top advice, I've now got my first coat of Hardwall setting... Got to build up to the level of the old wall, so it's going to need a 2nd coat before the skim. How long should I leave? The same 1-2 hours that you have to leave before the finish coat?

As CW says, it is due to suction.

I always second coat undercoat plasters after about 45 minutes depending on the thickness, suction and how warm it is. This fills the cracks as they form and bonds properly to the first coat.

think he's on about 2 pass on the backing jozeffo?...
do the second pass on the hardwall soon as the hardwalls picked up a bit, like jo says or youll have to scratch the first coat real well for a mechanical key...
surprised you havent managed to get the hardwall on in one coat at that depth though? or is it 25mm thick?

trick with backing coats is 'get it on, get it ruled and leave it alone' playing with it just brings all the water to the surface and it'll slump...

not so much with gypsum based backing but definately with sand and cement render, but the same still applies..

_________________
http://www.peterborough-plasterer.co.uk/

It's pretty thick...

Almost as thick as me really! I've only just realised that I've got a load of 9mm plasterboard in the garage which I would have stuck on if I'd known!

I need to go to about 15mm thick, but I've mixed up pretty thin in case the wall decided to continue playing silly buggers and suck out all the water again.

It's now on and ruled and I'm going to the pub so I can't carry on fiddling with it! Might skim in a couple of hours if I don't have too much to drink, otherwise I'll leave it and do it in a few days.

Thanks again for the invaluable advice guys.