New render crazing/cracking

S

sammoseley

Hi,
rendered half my new garage today, I was really happy with it till looked again an hour later and it started hairline cracking/crazing. Gave it a few doses of the mist spray from the hose over the next few hours but still cracking, even worse now. Pretty sure I should have wet it down more, eg. just after floating it.
I soaked the blocks beforehand and soaked the scratch coat which was nearly dry before putting top coat on.
I'm pretty sure the mix was right so I guess it just dried too quick, but I'm not too sure how I stop that happening next time ie. next weekend !!!!

background is medium density block.

Scratch coat 4:1 plastering sand:cement plus waterproofer, 8mm ish, soaked blockwork beforehand - did this yesterday

Top coat today 5:1:1 plastering sand, cement, lime 5mm ish, soaked scratch coat beforehand

didn't over work it, laid on, ruled off floated then final light rub with dry sponge, looked the business till it started cracking.

any advice much appreciated on how to stop the next lot cracking up

cheers
Sam
 
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S

sammoseley

sorry meant to add, scorching hot today plus wind which is probably the worst combination
 
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its been ok in hertfordshire mabey it was the wind heat and a combination of only being 5 mm thick top coat, usually i scratch coat it (yours sounds fine depth wise) then i top, rule then give it a seccond pass to build it out to required depth and level, so a combination of thin coat wind and heat is what i would say the problem is.
others might be along once thier out the pub ;) with more options mate, but without seeing it and what you have said thats what ive got it pegged at
 
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You needed to control the suction. PVA/SBR.
 
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Nothing wrong with PVA and render. You've been reading too much.
 
S

sammoseley

thanks for the replies, must admit never used PVA for external and always thought you weren't meant to but happy to be pursuaded otherwise. Blockwork has been up several months now in the hot weather so guess a couple of soakings with the hose before laying on wasnt enough.
PS, would you PVA before scratch coat, top coat or both.
 
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dnt pva, sbr it or you can buy external pva but i dnt really rate it prefere sbr.
usually if i need to i sbr before the scratch then with the waterproofer in the scratch coupled with the sbr base you shouldnt get any problems with the rate of suction
 
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I have used pva on outside rendering for a long time (more so now SBR because it is advailable and i dont have to pay for it. but you must remember when there was no pva or sbr we used to hose the walls down till they took no more water and that applied to inside and out. ) So I would put that down to part of your problem (not enough water to kill the suction), It seems like the sun or the wind is to blame aswell, if you render in the summer or when it is hot then you have to litterally flood the scratch with water until it doesn't absorbe any more water and when you top coat it, it doesn't pull the water out of the mix. Then you float it up as soon as you can get on it and sponge it, then you come back to it when it is picking up and you spray it with water and then (If you cant cover the work up) you come back when the sun has moved off the work and you spray it again. But if there is a wind on the work you must try to shield it and still keep it moist. Have a look at freshly rendered houses that have been done when it is hot
and sunny or when it has been windy and a few weeks later when it rains and it starts to dry it will look like a spiders web. This is the top 2 or 3 mm of the work drying out before the rest of the coat and that is the way it will stay and the thinner parts of the coat will crack widely and let water underneath it. The only way to cure that if it happens is to put 2 or 3 coats of good masonry paint on it when it is at the "cob-web" stage. Even if you have your mixes right for both coats it will happen, but it will only be the top coat that "cob-webs". The scratch coat is the important one and if you get that right and control the top one like i described then your on your way ;)
 
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dnt pva, sbr it or you can buy external pva but i dnt really rate it prefere sbr.
usually if i need to i sbr before the scratch then with the waterproofer in the scratch coupled with the sbr base you shouldnt get any problems with the rate of suction

We used it for 20 years before we were told it was a no-no. Never had a problem.
 
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dnt pva, sbr it or you can buy external pva but i dnt really rate it prefere sbr.
usually if i need to i sbr before the scratch then with the waterproofer in the scratch coupled with the sbr base you shouldnt get any problems with the rate of suction
i would just like to clear some thing up about external pva i have a tub on it it says only waterproof when mixed with cement so i guess its still water soluable unless you mix it into a cement slurry, i remember phoning them up last year because i wanted to use it on an internal application, and they said you can use it as normal pva but it has an extra quality of being waterproof when mixed into cement, other than that its the same as normal pva, heres a link to its specs scroll down the page http://www.amazon.co.uk/UniBond-Waterproof-Adhesive-Bonding-Exterior/dp/B003UYTM0W unless there are others out there that are different and waterproof without mixing it with cement it looks like if you just use it as a sealer it will still be water soluable
 
S

sammoseley

thanks a lot for the answers guys, I guess I'll have to see if it doesn't feel hollow then I might get away with a few coats of paint on it.
if not i'll re-do it
I'll go for SBR and a lot more on the hose for the next one,
cheers
Sam
 
S

sammoseley

PS I know you've already answered but thought I'd take pics just incase any further comment. It doesnt sound hollow when you knock it (yet !!!!)

the dodgy light coloured bits were when i wet it after it started cracking and tried to re-float to cover the cracks - bad idea!!!!

Cheers
Sam

View media item 34689 View media item 34688
 

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