Topcoat render won't stick

dex

New Member
This is war and peace I'm afraid, but please read and advise because it is driving me nuts.

I've built my own extension about 6 years ago out of block and put a sand/cement render finish on it. The first coat would have had water-proofer in it, the make of which I can't remember. Top coat would have been a weaker mix with an additive in, I may have used 'Feb'. The top coat crazed and cracked pretty quick but I have not bothered about it until now.

Recently I knocked off the top coat, came off quite easily and the scratch coat is left. This is solid - no cracks or anything. Don't want to take off this as it will damage the blocks underneath.

I did a bit of reading on this forum. Primers were mentioned so I have painted all the walls with a pre-grit primer, from Bond-It. I then put a top coat mix on one wall, 5 plastering sand, 1 cement and 1 lime. It didn't seem right when I was putting the mix on, very sandy not sticking like it should. Next morning nearly everywhere was hollow so knocked the whole lot off. I power washed the wall and tried another mix on this wall yesterday. Mix was 5 plastering sand, 1 cement and 1 lime with a dash of plasticiser. I dampened the wall down first and mixed in the mixer for longer. I noticed when putting it on , that in places there were air bubbles underneath the render. This morning there are already cracks over the wall and hollow sound when tapped. This lot will have to come off again. What is going wrong here? It's obviously not sticking to the scratch coat but I don't know why. Any suggestions what to use instead of sand cement render?
 

dex

New Member
Thanks for the replies. I will try the sbr cement slurry. Just one more question- what does a "dead mix" mean
 

Beenereb4

Active Member
You are maybe putting in to much water to the mix which will give you a dead mix, start with a little water and let it mix for a couple of minutes before adding more water, the mix will then be really creamy and sticky.
The problem may be that you are charging the mixes through to quickly with too much water and when the water is drying out you are getting alot of shrinkage causing your failure.
Let me know if this helps.
 

Beenereb4

Active Member
As a guide about a third of a bucket of water will be enough for twice your gauge depending on how wet your sand is.
Also grab a handful of your damp sand it should almost hold itself together if it doesn't don't bother using it.
 
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