outside rendering

Is there a DPC in the wall? Did the render get frosted?

I think it was because you used waterproofer and did the second coat a week later. After a week the render is probably fully cured and ther will be no suction available for the second coat because the waterproofer wil not allow it. I'm no expert so its just a guess.

Sorry to hear about your wasted work. 3 months is certainly getting on a bit for cement. The mix was maybe a bit on the strong side too, but I don't expect either of these is the main reason for the failure.

You should ideally apply the second coat the next day, or sometimes the same day in warm weather if the first coat has set and stopped shrinking. Leaving it for a week means the first coat probably became too dry. Did you wet the surface really thoroughly? It has to be wet because otherwise the dry surface sucks the moisture out of the new render and kills the setting process, and then you end up with a thin powdery layer between the two coats.

Pete near High Wycombe :o)

Had exactly the same problem about 2 years ago. Bricked up a window and had to rended the surface to match the house. Problem was the render had to be about 1 inch thick ( given my inset bricks) so I put it on in two layers..First layer went on fine...Left it for about 4 weeks during which time it also got a bit dusty, then got around to doing the second before chucking lots of wee pebbles at it to match the house. 2 weeks later I thought it looked blown and tapped it..HOLLOW sound. prodded away and the whole 2 feet by 4 feet lump slid off! Kind of realised what I had done wrong..Got out watered down PVA solution and soaked the wall in it..Really soaked the wall until it stopped drying out quickly then re-did the render.. Suffice to say its still up there!

Ged

Sand and cement should really be recoated in 24-48 hours (might stretch a bit longer if it's cold). Any longer, and it needs to be handled not as a recoating, but as a scratch (base) coat application, as you did.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick. There was no DPC and no frost at the time.

Many thanks for the prompt advice I will take it back to the concrete blocks next year and apply a single layer of rendering to be on the safe side. Fresh cement and a weaker mix with waterproofer should do the trick.