They are temperamental and require temperatures around 75 degrees. Polyester resins are naturally resistant to ultraviolet light and are used more widely than any other type of resin. They are less troublesome than epoxies, but also require careful mixing and ambient temperatures. Vinyl Ester Resins are typically used in applications requiring corrosion resistance including building and repairing chemical storage tanks. Vinyl Esters are often described as a cross between polyester and epoxy as it falls between them in both price and handling characteristics.
Gel coats are special polyester resins which are generally used on both part and mold surfaces.
Gel coats are best used in a mold and are not recommended for surface repairs where paint would be suitable. Similar epoxy materials are generally called Surface Coats. Applying the resin to a surface is best accomplished with mohair rollers or a brush. Working the resin into the reinforcement requires saturation rollers or squeegees. Spraying is usually recommended for gel coats and large jobs.