Polyester resins and gelcoat have a limited shelf life, and will eventually harden in the container—without the addition of catalyst. Shelf life depends on several factors, such as product formulation and storage conditions. In general, the shelf life for uncatalyzed polyester resins should be up to 6 months, when stored in dry, cool conditions below 70°F. If stored properly, the shelf life may also exceed this, but always check to see if the material has thickened, become clumpy, or otherwise polymerized before using.