If you apply a finishing coat over a backing coat that has dried completely, the new coat will adhere poorly and its shrinkage will be resisted by the rigid undercoat, causing it to crack.
Plastering is both an art and a science. One of the most common and frustrating problems faced by DIY enthusiasts and professionals alike is cracking as the plaster dries. While a few hairline cracks can be normal, excessive or deep cracking indicates an underlying issue. Understanding the science of drying is the first step to achieving a flawless, smooth wall. plaster cracking when drying
Over a taped plasterboard joint or recurring crack, apply self-adhesive fiberglass mesh tape before filling. If you apply a finishing coat over a