Desperate for advice on blown render!!!

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davidtaylor849

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Hi folks. I am new here and i am hoping to get some help and advice on my rendering.

nutshell story so far:

7Newton block garden wall, 3 blocks high. The wall was cleaned, prepped and given a coat of PVA. I applied the scratch coat at a ratio of 4:1 and added 150ml of water proofer to the mix. keyed the scratch coat and let it set for a couple of days..... ALL GOOD to this point. Scratch coat is firm and well set.

NOW - Top coat: Here is what i did: Dusted of the scratch coat to get rid of any bobbly bits that might get in the top coat when applied. I gave the wall a wet down (not too much - just enough so that it just starts to 'give back' a little water). Mixed up 5:1 rendering/plastering sand to cement and again added 150ml of waterproofer to the mix. (as the area is not too large, i mix a back of sand at a time which requires 30:1 ratio of waterproofer to water - hence the 150ml of waterproofer).

ANYWAY - i applied the mix to the wall and all looked good, i began to rub up the render when my finger could feel the resistance in the render as it began to set. That seemed fine and the wall looked great! - - - - BUT......
36 or so hours later, i got that 'not quite right' noise when i tapped the render with my finger nail...........YES it had blown (only in places), unfortunately FAR too many places :-(

Out comes the chisel and sure enough the render just fell away from the wall in chunks. Now like i said, some areas are fine, the render sucked down like beautifully like Devine Brown on Hugh Grant. Unfortunately though, not many of these areas existed.

So i say to all you good people. - - HELP ME PLEASE. What am i doing wrong?

The weather was good and sunny when i did the top coat, i sprayed water on the coat to keep it from curing too quickly (every 2 hours or so).

I am getting tired of going back to Wickes and buying sand, then waking out with several of the oh so friendly shop assistants taking bets on whether "his render will blow again - i'll take some of that action" type comments. :-)

Thanks to all of you that can help me save my embarrassment.
Cheers
Dave
 
Not saying its the reason but i wouldnt have damped down the scratch coat or put waterproofer in the second coat, you may have worked the second coat to much or got on it to early
 
wicks sand it is shite you need angular shape particals . How soon was it before you could rub up try bq render sand by hansons.
 
What has blown the scratch coat or just the top coat, henry is right wickes sand is shite its not sharp enough that said it should'nt have blown. Did you use fresh cement. Theres no need for waterproofer in the top coat but put some plasticizer in
 
i reckon pva aswell, but as has been said before some people have been doin this for years and not had the problem
 
wickes don't sell plastering sand mate

actually they do acccording to google but I've never seen any round here.

First get the right kinda sand.
 
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:RpS_w00t:TOP COAT A good 5:1:1 , well graded wahed,cement and lime. Small amount of wateprrofer which has good air entraining properties.Do not wet the scratch unless very large area. Make sure the top ct is not to thickly applied 6 to 8mm max., in general thinner than the scratch. Your should pay particular attention to the eveness of your scratch coat,so your top goes on even depth all over..You may have overworked thicher parts of top coat whuch may be the reason for certain areas de laminating.did you key the scratch coat well enough for a good mechanical key.
 
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