Compare the strength of epoxy to fiberglass. These two systems are very different. Polyester resin and fiberglass cloth verses epoxy resin and fiberglass cloth gives a very similar looking product. Epoxy forms a vapor barrier when cured while polyester does not These differences have a substantial impact on boat building and repairing. The greater flexibility of epoxy allows the sheathing to move with the hull, as it works under loads.
Polyester is stiff and rigid.
After a short life, it shows signs of fatigue and cracks when forced to flex. In addition epoxy's high resistance to scratching gives it greater durability. The third difference between the two, is that epoxy is a strong adhesive, while polyester is not an adhesive. This is important as it is this property that ensures that the skin of strong cloth remains bonded to the hull. Finally epoxy's property of forming a vapor barrier on cure is essential to fully understanding the difference between epoxy and polyester.