I did a job about 12mm on rend-aid in April last year. A couple of places that may have been a bit thicker and got some slump cracks. Filled in on rubbing up but opened up again by next morning. Most of my work is 2 coat being re-furbishment so about 12 mm max on rend-aid.
PFT Wales had a good tip. If it is late in the day put some droppings in a sealed plastic bag or if earlier in the day mix a bit at night and put in a sealed bag and place in a fridge. Just right for crack filling next day.
Don't get it Irish Spread? Either me or you has mis understood EssexAndy.
I've never had a problem with OCR slumping or shrinkage cracks, and have gone on 20mm before maybe a bit thicker.
Very good render, generally I do same a Rigs RendAid then 10mm beads one coat.