render not sticking

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hi all, external wall of breeze block, successful scratch coat dried and suck perfectly. 2nd coat initially at 4:1 after drying came away from the wall in sheets when dry. I thought the sand cement mix was not strong enough so re-did again 2nd coat at 3:1 but this is also starting to sound hollow when tapped as it dries.
i've done this before but don't understand why it wont stick. The back side of the render that's come off has a sandy dusty finish. Using a float and pushing hard to get the mix to stick, building sand at 8mm using a plastersiser onto a rough scratch coat.
I made sue i put it on on a cloudy day and covered it to slow the drying down

I have rendered a 6m*3m wall as part of the same job and its stuck properly.
can anyone help?
should i be throwing the topcoat on, is a float enough pressure?
really frustrated!
 
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sorry i mean not sharp sand or very fine.

maybe its dried out too quickly, its not consistent maybe i should have kept the work damp for days after?
 
Your mix is too strong ,the scratch coat i normally do at 4-1 and then 5-1 for the top coat ,the top coat being stronger than the scratch coat is what is causing your problem
 
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ok thanks, ive obviously gone the wrong way, i understood more cement in the mix to be stronger, il do a test area at 5:1 and let it dry slowly. thanks for your help.
 
How much plasticizer did you use in a mix ?, too much will make the scratch coat set soft and dusty, hence the top coat will not bond to it.
 
i put in 100ml per 10litres as it said. scratch coat is hard.
im thinking the 4:1 should have been 5:1 as earlier advice and to keep it covered and go back and spray to keep damp over several days after finishing
thanks
 
Puttiing a strong coat on top of a weaker coat is not good in the long term, but it would not be the cause of the coat not sticking. If anything, a stronger coat is more likely to stick, all other things being equal. Think of how sticky a 1:1 coat is compared to a 6:1 .

The cause here is something different, some of which have been mentioned and you have eliminated

- the surface of the scratch coat
the rate of second coat drying out (too much heat, too much suction)
quality of mix ( sand too soft, too much plas, overmiixed, combination of any of these)

The sand on the back of the coat may even indicate that it was too 'dead' - does it 'sit up' on your spot board or on your hawk? When you put your trowel through it does it show the sand grains?
 
well here go's agen
scrach coat 4:1 with waterproofer not plastersizer
wet the wall first with a hose before each coat
or use a cement slury or EVA
dont use waterpoofer in the top coat
scrach the first coat with a render comb
top coat 5:1 mix or 5:1:1 sand cement lime
building sand is fine for rendeering
plastering sand is good too
 
troweladdict";p="1689726 said:
well here go's agen
scrach coat 4:1 with waterproofer not plastersizer
wet the wall first with a hose before each coat
or use a cement slury or EVA
dont use waterpoofer in the top coat
scrach the first coat with a render comb
top coat 5:1 mix or 5:1:1 sand cement lime
building sand is fine for rendeering###########
plastering sand is good too[/quote

####I personally would not use building sand for rendering, brick or block laying yes..Rendering No....
 
roy c";p="1689741 said:
well here go's agen
scrach coat 4:1 with waterproofer not plastersizer
wet the wall first with a hose before each coat
or use a cement slury or EVA
dont use waterpoofer in the top coat
scrach the first coat with a render comb
top coat 5:1 mix or 5:1:1 sand cement lime
building sand is fine for rendeering###########
plastering sand is good too[/quote

####I personally would not use building sand for rendering, brick or block laying yes..Rendering No....

Nor me Roy!! use proper washed plastering sand for rendering,,, always.
 
Thanks for each of your advice, im not in the building game but do appreciate the obvious skill to adapt and interpret different situations. I know the top coat needs to have less cement and that it needs to dry out slowly.
I have taken what has been said and put a 5:1 top coat on to a wet rough scratch. This will dry out over the next few days and il let you know the outcome.
 
do you ever mix your sands ? say 2 sharp 1 soft, or 2 plastering 1 soft?

Steve, yes , if it is a too coarse. Sometimes it's a little dead or greedy and a bit of soft in the mix gives it a bit of life.

You're still not making the overall aggregate too soft though if you just put enough in - but as you know that depends on the sand .

Sometimes we get a batch of 'Leighton Buzzard' that's very dead and a few shovels of soft does the trick.
 

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