The resins specifically designed for SMC/BMC applications are thermosetting unsaturated polyesters (UPR), engineered specifically for high-pressure molding (140–160°C, 50–150 bar). Standard resins, conversely, are typically formulated for hand lay-up or pultrusion processes; they exhibit high shrinkage and poor thermal stability, making them unsuitable for high-pressure, rapid-curing environments.

They are not entirely interchangeable.
SMC Resin: Characterized by low viscosity, a rapid thickening rate, low shrinkage, and excellent wet-out properties for long glass fibers; it is used in Sheet Molding Compounds.
BMC Resin: Characterized by medium-to-high viscosity, high filler loading capacity, excellent dispersion of short glass fibers, and rapid curing kinetics; it is used in Bulk Molding Compounds.
Consequences of Intermixing: Using BMC resin in an SMC application results in poor flow characteristics and potential material shortfalls (incomplete mold filling); conversely, using SMC resin in a BMC application leads to slow thickening, mold sticking, and reduced mechanical strength.