You can pour the next layer after the previous pour has gone through its heat cycle, which is usually around 24 hours. The rule of thumb is if you can make a imprint with your finger nail in the epoxy you can pour again and the epoxy will chemically bond to the previous layer. If you’ve passed that point, sand the surface of the previous layer with a high grit sand paper (180-220 grit). This creates a rough surface for the next layer of epoxy to stick to. Once sanded, clean the surface with an alcohol based cleaner like de-natured alcohol to remove all the dust from sanding. After that, you’re good to pour!