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.