Ahhhhh... Yep, over 35 and things go pearshaped.. Had this happen to another customer who lives in Western NSW, so your experience confirms my theory.
The humidity a few days after the heat shouldnt have too much bearing on the finish, however had it been that humid when applying you could have problems as well.

Thanks for the info, and if you run out of finish coat flick me an email and I will send more out to you free of charge.