Top coat sand and cement render not adhering

13 Feb 2009
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United Kingdom
External rendering onto lightweight aerated blocks. Yesterday wetted the wall, applied sbr slurry, waited for it to go tacky, then applied 6mm scratch coat (5:1 plastering sand to cement with plasticiser and waterproofer).

This morning scratch coat was rock solid, so wetted it down and applied top coat (6:1 plastering sand to cement with plasticiser).

The top coat isn't sticking well and slumps in places too.

Mixes are deliberately weak due to the strength of the aerated blocks.

Is there another additive I should use in the top coat mix, to make it stick (I've assumed plasticiser replaces lime)?

Final decorative finish to be tyrolean, if that makes any difference.
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