Liquid polyurethane plastics are moisture sensitive, so it is important to only add dry fillers to these resins. Foaming occurs when fillers have moisture, or the resin container has been opened long enough for moist air to get in and contaminate the liquid. For this reason it is important to keep the containers closed when not in use, especially in the summer when the humidity is higher.
More Information: https://www.polytek.com/tutorial/adding-fillers-polyurethane-casting-resin