Polyester resin is extremely rigid, which makes it very brittle. Polyester holds are notorious for breaking by merely tightening the bolt. The strength of Polyester cannot compare with the strength of Polyurethane. Of all Thermoset plastics, polyester resin is the cheapest and poorest quality. It was never designed for making climbing holds. If you see really cheap holds for sale, they are probably Polyester. Polyester resin, can be purchased for about $7 per gallon. We spend about $40 per gallon on our polyurethane. The difference in quality and safety is well worth it. Three Ball Climbing Holds are made in the USA with Polyurethane that was developed specifically for making climbing holds. This polyurethane is also rigid, but it can flex if it is fastened to the wall with extreme pressure. Our polyurethane is non-toxic, environmentally friendly, and lead free! Click Here for more info on Polyester Resin and Polyurethane.