Polyester used in casting is a type of alkyd, which is a synthetic resin made from polyhydric alcohols and polybasic acids. Alkyds are used in a variety of applications, including paints, coatings, and adhesives. In casting, polyester is used as a binder to hold the components of the cast together. It is also used to create a smooth surface finish and to reduce shrinkage during the curing process. Polyester used in casting is highly durable and resistant to corrosion, making it an ideal choice for many casting projects.