Unsaturated polyester resins (UPR) were first developed in the late 1920s in the research group of Dr. Wallace Carothers at DuPont. In the late 1940s, coatings made from these resins were applied to molds to protect them during the fabrication process of fiber-reinforced composites. By the early 1950s, several companies were offering pigmented gel coats for sale.

Today, gel coats are a common surface finish, perhaps best known for their ability to improve product appearance, such as the surface of a boat hull. But gel coats are also used to improve weathering, filter out ultraviolet radiation, add flame resistance, provide a thermal barrier, improve chemical and abrasion resistance and provide a moisture barrier. Let’s take a closer look at gel coats.

A Chemical Description

Chemically, gel coat formulations comprise building blocks from five classes of raw materials: resins, pigments, additives, monomers and fillers. The resin impacts almost all application and in-service properties. Gel coats can be formulated from many types of resins, including UPR, vinyl ester, acrylics, epoxies and urethanes.

Colorants and pigments impact color, water and UV-resistance, opacity and so on. Additive packages are used to affect flow and leveling properties, curing, UV-resistance and more. Monomers are used in gel coats as reactive solvents. During the curing process, the monomer reacts with the polymer in the base resin to form the thermoset backbone of the gel coat. Finally, fillers impact spray characteristics, fire retardancy, density and flow properties.

Table 1

Types of Gel Coats and Related Products

A comprehensive list of gel coat types and ancillary products is shown here.

Standard Gel Coat Products

Specialty Gel Coats

Ancillary Products

Sanitary gel coats

Transportation gel coats

Marine grade gel coats

Wind energy gel coats

Tooling gel coats

Clear gel coats (marine)

Clear gel coats (marble)

Surface coats

Sanding primer gel coats

Vinyl ester barrier gel coats

Fire retardant gel coats

Conductive gel coats

Tie coats for epoxies


Patching compounds

Wax solutions

UV absorber solutions

Inhibitor solutions

Promoter packages

A Functional Description

Functionally, a gel coat is a high-quality finish designed to provide great aesthetics (color, gloss, opacity) as well as UV protection and water resistance to an FRP composite. The gel coat is generally applied to a smooth mold with a release agent. A special class of gel coats known as surface coats can be post-applied to existing parts.

By changing the building blocks in a gel coat, a skilled formulator can develop products for a wide range of end-use applications. A comprehensive list of gel coat types and ancillary products is shown in Table 1.