"alanjones" wrote in message
oups.com...
I built a concrete block wall this summer and rendered it about a month
ago. I applied 2 layers of render the first was well scratched before
the second was applied a week later. I used sand:cement 4:1 with
waterproofer 1:25.
About a week ago the top later detatched itself from the base layer and
fell off. The other walls I did are also losing thier top coat (they
sound hollow).
What did I do wrong?
Was the cement too old (about 3 months)?
Did I over float the top coat to get it level?
I thought I'd done a very nice job and wish I'd stuck at one coat of
render!

Many thanks Alan of Exeter.


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 )