RENDERING trouble

hi

rendered the inside of slightly an old brick wall [shed] , couple of days later the top coat cracked and is hollow. the weather was hot but wall was shaded .

used 4 to 1 . [no plasticisor or very little] odd thing is that a specialist cement work web site recommends builders sand but the w.h. smiths diy book says builders sand will cause cracking, so im very interested to know why there is a disagreement here as local builders always use builders sand [not plasterers sand]

haven't looked at scratch coat yet but it looked ok before we applied top coat, but there's another query, as we used no pva on the brick wall will the scratch coat come off

can anyone advise me
thanks

 

test

 

thanks Mr. HandyAndy