Overlay is a situation that is best avoided. Under circumstances that are not ideal, mycelium is able to overtake the casing layer. This overly colonized casing layer is called overlay.
In this case it is difficult for the mycelium to pin, rendering the casing layer useless.
Overlay can be prevented by introducing the mycelium with the added casing layer to fruiting conditions at the right time. After adding the casing layer the substrate needs to be put back in the incubation room for some days. When the mycelium is visible on 20 to 30 percent of the surface, introduce the mycelium to fruiting conditions. Cold shock can have a positive influence in preventing overlay.