Render- what am I doing wrong?

[Corporal Jones]

2 weeks ago I scratch coated a brick boundary wall, 2 x 5 mtr, 1 to 4 mix (50 year old render fell off) no problems with it apart from an aching back. Today got on with the final coat, 1 to 5 mix, applied around 6 m2 and went in to get a drink, came outside again and saw that about 3/4 had fallen of the wall so went back in for a stronger drink to ponder the situation. Any ideas? I scratched the first coat so did not use PVA and hosed down the wall, I am wondering if I counted the wrong mix of cement as I was distracted at the time (no not with a busty girl walking by but the swmbo, mind you she is not short in that department but that is sadly a distant memory)

[bluesky]

Possibly too hot a day, direct bright (hot) sun etc

Assume yr using opc/sand not lime/sand

[mark]

What I do is get a bucket of water and a large paint brush and flick water on to the area to be rendered. This stops the wall being thirsty and sucking the moisture out of the new render.

mark

[Corporal Jones]

Using cement/sand, only up to 16c today, sun has only just got round to the wall so given up to the whiskey now for the rest of the evening.

[Corporal Jones]

Gave it a good hose down before I started.

[Andrew Gabriel]

That problem is existing render too dry, and has sucked too much water out of the finish coat. A second problem (which you didn't get as far as hitting) is that the finish coat probably won't bond very well to the scratch coat.

For sand and cement, finish coat should be done 24-48 hours after scratch coat. This is for two reasons - the scratch coat moisture level will be about right for the finish coat to stick, and the scratch coat is still setting and will cross-bond with the finish coat.

At this point, I suspect you'll need to do it as for a reskim. That means water to kill the suction in the scratch coat, probably mixed with a bonding agent to help the finish coat stick to the scratch coat. I'm not sure what bonding agent to suggest. PVA is water soluable so might not last for too long outside. EVA (Exterior PVA) is more water resistant, but only in a cement matrix (not as a bare glue), so it might work. SBR is probably too waterproof and might lock moisture in the wall. A cement slurry is sometimes used as a bonding agent. Maybe someone else can suggest the right bonding agent?

[stuart noble]

IME pva is fine. It doesn't need to be waterproof as its only purpose is to partially seal the scratch coat long enough for the final coat to set. If you overseal, the next coat tends to slide initially but, once suction takes over, it hangs on ok

[Corporal Jones]

OK will cover with PVA tomorrow & have another go!

[Andrew Gabriel]

That first coat should be diluted, about 4 parts water to 1 part PVA. Let it soak in and dry if you're following it with...

This is a cement slurry bond, which should work OK. Personally, I don't use sand in it though.