Wetcast in situ,want to smooth render but scratch coat fails

[hopern1]

So the house I live in has been wetcast and I hate it. I would like to have it smooth rendered.
My plasterer, nice chap, works hard, but is relatively young, has tried and failed to smooth render it.
He PVA'd it for adhesion, but the scratch coat he was trying to apply simply fell of.
He reckons it was falling off because of the relatively new Dulux Weathershield masonry paint. So he tried to key the surface a bit by taking a shovel to it. No good. The wetcast is solid as a rock. Would be easier to knock the wall down (not an option obviously).
Any suggestions ?

regards
Norris

wetcast in detail

• hopern1
• 27 Jun 2012
• 1

gable end of wetcast

• hopern1
• 27 Jun 2012
• 1

 

[Micilin]

Hi,

Your young man is probably very good, just lacking a bit of experience.

He should not be using PVA outside - and to be fair I'm not sure how much of a chemical key you need on roughcast that wil provide a good physical one. I wouls use SBR to seal or beter still an SBR/OPC/Sand slurry (use the search button for more info) I suspect the PVA was dry before he got the render on?

I'm not sure how the scratch coat 'simply fell off' - was it as he was putting it on, did it sag after or did it just not adhere when set eg sound hollow?

It's nothing to do with the paint, if the paint is sound and roughcast is sound.

I'm afraid it sounds like he is having a go at something that he has no experience in - there should be no problem rendering over roughcast.

What were his mixes, (they may hve been in the mixer too long, or too much feb) what was the weather like and how thick was he putting it on?

The bits you took off- are they solid or crumbly?

 

[hopern1]

Thanks Micilin
so a few questions there.

PVA was dry before he got the render on ? I was there and I don't believe so.

Literally the sctratch coat fell off, he applied it, and it just fell off the wall. It wouldn't stick, which I was very surprised at, as the surface is clean, true, solid and VERY rough texture.

I can't confirm the mix , how long it was in the mixer or if too much feb. It did look very wet to me, but I am a lay man with no experience of this.

The surface is sound, the most that came off when trying to key the surface was some of the more prominent stones on the wall broke (yep the wetcast/roughcast is so solid, when you hit it, the stones break instead of dislodging from the wall).

regards
Norris

 

[Micilin]

Perhaps he was not able to push the render onto the wall .

Hard to explain (help needed from others !!) but I see a lot of spreads who 'place' the render/plaster against the wall without pushing it onto the wall.

With roughcast the background he defenitly needed to be able to force the render onto the wall rather than just getting it on it.

Perhaps he was puttingon too heavy /thick a coat and it slumped. It may also not have been rich enough , or a 'dead' mix that would not 'sit up' on your hawk . Very hard to see why but can certaihly say it should not be happening.
If he pva'd it with a brush, it rules out dust.

Perhaps it might be easier for an unven surface to harl/dash/scud render on first , then flatten it, scratch it, then apply another scratch coat normally .


Edit-

I think it probably was too heavy/thick and slumped off. An initla coat for a key, then a scratch coat (s) to even out then float coat wold be better

 

[peaps2]

I would attack the cast with a grinder for key, paint won't help matters, neither will the pva IMO. i suspect Micilin is correct in saying the scratch coat was too heavy and without a good key it was bound to fail.

 

[roy c]

I agree that aside from hacking all the rough cast off it needs a phsyical key which would mean grinding diamond patterns thru the rough cast with a sthil saw grinder.Then horizontal lines thru the diamond pattern.Then you will have to "scut" it with a sbr slurry that micilin reccomends. Then after the background has been prepped then it will be ready for scratching.You have got to remember after it has been prepped,scratched, and floated you would of added another 1"or more depending on the dash used before, how will this effect window and door revels soffits and abutments? There is a lot to take into account when rendering over dash or wet cast and that is where experience comes in....