Rendering onto concretre

Members online

Status
Not open for further replies.

solway

Active Member
Hi all,small bit of a problem.About a month ago myself and my mate took on to plaster a concrete wall about 12ft high by 20ft long. We maintained that a good polybond scud,scratch coat and finishing coat would be an ideal method of work for the said area. We mixed the scud at 2 sand to 1 cement with polybond mixed through.The concrete wall was dampened before the scudding. We scratched it 2 days later with a scratch coat of 4 sand to 1 cement.Dampened wall prior to plastering with water as normal. 4 days later we applied the finish coat at 4 sand,1 cement,1/2 lime. All went smoothly until I was passing it the other day and saw a few cracks appearing in it. I stopped and investigated more and there is a hollow sound which tells me the render did not key as it should have. I removed a small area and the scratch coat seems not to have keyed to the scudding coat.The scudding coat is "welded" to the concrete but the scratch coat seems to have only been "resting" on the scud.It came off no problem at all.Can someone point me in the right direction as to what we did wrong as it is driving me crazy every time I pass it out.Should we have applied a bonding liquid to the cured scud coat before the scratch coat or.......?we have done other areas in the same manner before and havnt had any problems. Im very interested to hear peoples solutions and reasons
 
I don't think you'd have any problems if you used SBR dude.. if scratch applied when still tacky then you would essentially have 'welded' the scratch to the wall too. Best option for sure!
 
A good scud coat with sbr would of done the trick,next day scratch with four to one, leave for a week and float with five to one one sand, cement lime. Mixing sbr in the scratch would help too, also fibres in the scratch and top coat
 
I think grit in the scud would have helped with the key. Rend aid is best tho IMO, it takes a great grip and sets very hard!
 
I generally (sbr+ cement) mix a small area then get a scratch coat over it before it dry's they both set together ,works for me :RpS_thumbsup:
 
Hi all,small bit of a problem.About a month ago myself and my mate took on to plaster a concrete wall about 12ft high by 20ft long. We maintained that a good polybond scud,scratch coat and finishing coat would be an ideal method of work for the said area. We mixed the scud at 2 sand to 1 cement with polybond mixed through.The concrete wall was dampened before the scudding. We scratched it 2 days later with a scratch coat of 4 sand to 1 cement.Dampened wall prior to plastering with water as normal. 4 days later we applied the finish coat at 4 sand,1 cement,1/2 lime. All went smoothly until I was passing it the other day and saw a few cracks appearing in it. I stopped and investigated more and there is a hollow sound which tells me the render did not key as it should have. I removed a small area and the scratch coat seems not to have keyed to the scudding coat.The scudding coat is "welded" to the concrete but the scratch coat seems to have only been "resting" on the scud.It came off no problem at all.Can someone point me in the right direction as to what we did wrong as it is driving me crazy every time I pass it out.Should we have applied a bonding liquid to the cured scud coat before the scratch coat or.......?we have done other areas in the same manner before and havnt had any problems. Im very interested to hear peoples solutions and reasons

Not quite sure what a scud coat is , but we always apply a stipple coat of 2-1 with 50,% pva on to the concrete wall dabbed on with a stiff hand brush , leave for at least 24 hrs damped down then apply a scratch coat of 4 - 1
 
Not quite sure what a scud coat is , but we always apply a stipple coat of 2-1 with 50,% pva on to the concrete wall dabbed on with a stiff hand brush , leave for at least 24 hrs damped down then apply a scratch coat of 4 - 1
Also wrong .dont put this in a book..
 
Still no wiser as to what a scud coat is :RpS_confused:
A scud coat to myself is a mix of sand and cement at the ratio of 2 or 3 to one, in the water which I mix water 3 sbr 1 . I then mix the sand cement and water to a sloppy consistency, this I then splatter onto the walls covering every,square inch, ps no pva outside ever.
 
Thought it might be along those lines, its what we call a splatter dash or stipple coat, seems it is local terminology , that's one of the problems with the plastering trade different terminology to many different jobs . I the first time I went to work in London I was asked to set a ceiling, didn't have clue what he was on abour , we call it skimming
 
Thought it might be along those lines, its what we call a splatter dash or stipple coat, seems it is local terminology , that's one of the problems with the plastering trade different terminology to many different jobs . I the first time I went to work in London I was asked to set a ceiling, didn't have clue what he was on abour , we call it skimming
Of course there are always regional variations of terminology, my pal in Australia was refused a job from another Irishman because he did not know what sheet rocking was , my pal,was a carpenter ( chippy) he calls dust sheets (drop cloths) Americanism I think.skimming when I was,an apprentice was (hardwall ing) plasterboarding was (slabbing)
 
what we call tacking, in the west country it was boarding. to float in sand and cement was greening, setting was pinking
 
I've worked in Essex with other plasterers from Essex pretty much my entire career and have heard all of the above names for the different aspects of our trade except for boarding/tacking being refereed to as slabbing. Heard that for the first time when I employed my mate from Ireland.
 
I would of applied a slurry coat :) one day,applied another the day afterthen scratched on while tacky.
a bit like like over skimming really,i always PVA then skim while still tacky NEVER when PVA has dried
Plus even plasterers I work with around here work different to each other,but mines the best way :)
 
Pva is no good for outside render it just forms a skin on the wall you need to mix sbr and cement together and stipple it to the wall and then scratch coat it whilst still tacky and if in doubt add a splash of sbr to the scratch coat mix as well.
That's what I've always done and never had a problem
 
Pva is no good for outside render it just forms a skin on the wall you need to mix sbr and cement together and stipple it to the wall and then scratch coat it whilst still tacky and if in doubt add a splash of sbr to the scratch coat mix as well.
That's what I've always done and never had a problem
Yes but mikey, ive always used PVA on rendering for nigh on 30 years and erm, never had a problem. Ive wet many scratch coats down with pva to kill suction before dashing( cos half thr time waterproofer is usually shite)
Shirley that can't just luck ?
 
Yes but mikey, ive always used PVA on rendering for nigh on 30 years and erm, never had a problem. Ive wet many scratch coats down with pva to kill suction before dashing( cos half thr time waterproofer is usually shite)
Shirley that can't just luck ?
I was a sinner too for yrs Craig, only in the last few have I started using sbr, may god forgive me for I knew no better, never had any comebacks either though
 
I was a sinner too for yrs Craig, only in the last few have I started using sbr, may god forgive me for I knew no better, never had any comebacks either though
To be honest John,I use SBR now,I love the stuff,BUT if I'm in the middle of nowhere and I have PVA and no SBR,I would use it without a second thought :)
 
I'm not Gona knock you for using pva outside Craigw but I've been taught by me dad who's been in the game for 35 years and never had a come back so I'd never use it just rig a hose up for them shitty thermolite blocks and keep soaking them or if its cast concrete use sbr can't go wrong then leave the pva in the tub mate .
as for waterproofer never put it in render causes damp problems inside the building later down the line mate
Cheers
 
Status
Not open for further replies.
Top