Been Plastering 13 years done loads of sand and cement rendering. The way I have been taught is that some times render just cracks?? I’ve done loads of jobs and 80% of them have never cracked. Just done a small extension and it’s cracked even though I put all the right stuff in it put it at the right thickness was on concreat blocks it’s still cracked why is this and how do I put this right? My idea is to k-rend over the top of it if I do so will I need to keep up the render or can I just pva it etc???