essexandy
The Lake Governor
It could be a number of things but i think without a good key it will fail and skimming on a smooth render i think your asking for trouble, also a sandy render you will have problems from the off and like you say shrinkage will cause problems if you get on it too early. Every time ive seen finish fail on render the key has been the problem coz it aint usually there or it aint deep enough.
If you saw my scratch float you'd be horrified then lol. The screws stick through no more than 2mm at a guess.