SBR over crazy render?!

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Hiya...
I cement rendered a top coat on my thermoblock extension yesterday, but the top half has crazed patches.
The bottom half, completed while weather was cloudy, has turned out fine so far, but as soon as sun hit the top later, it went off extremely quickly, and started cracking despite sponging and misting regularly.
It's 4:1 over tacky(ish) SBR slurry on dampened scratch coat. Some small patches have bubbled a bit, but sit back in place undetectably when tapped, they're mostly less than 1-2 inches diameter.
I busted my ladyballs to get it done so would really rather not have to start again if it can be avoided!
My questions are:
Is the cracking likely to be due to the heat, or is it possible i overworked it? Or something else?
And, is putting a coat of SBR over it before painting worth a try? (I tried this on one of the worst patches and it's stabilized it to a nice solid surface, but is it a stupid idea or likely to fail horribly at some point imminently?)
Thank you.
 

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