For thick-walled components which were cast in several steps it can lead to delamination.
Please consider the following casting multi-layered build up:

1. The best solution is to pour on the not yet completely cured surface of the previous layer. That means the previous layer should still be sticky.

2. For hardened layers the layer to cast on should first be roughened and made free of dust and grease before another layer can be casted on.