scratch coated a pine end 3.1 mix on to a dry pva cement wash 2.1 fine sprayed areas before laying into able to get 2/3 bays on before going back to float finish the sun was on it for about 2/3 hours before it went over roof , checked over for 2/3 days no hollow sound all good on 4th day all went hollow and coming off in sheets, all pine end 50m2 found that the back of the render coming off had a slight dust latence on back of it pretty sure this is why it didnt bond anyone has had the same probs during this hot spell . just done test panel laying into a wet pva seal hoping this will do the trick.