gelcoat (a foreign word that in English is composed of gel and coat, which accounts for its gel appearance and its use as a coating) is a material created to provide a high-quality finish on the visible surface of a fiber-reinforced composite material. cross-linked and subsequently covered with composite polymer matrices, which are usually mixtures of polyester resin and fiberglass, or epoxy resin and glass.

The manufactured component, when it has fully cured and has been removed from the mold, presents the finish conferred by the gelcoat. Pigments are usually used with the gelcoat, giving smooth, shiny and colorful surface finishes that increase the aesthetic appearance of the product, as happens in solid surfaces and polymeric imitations of granite or marble.

Many naval boats and aircraft are made using composite materials that have an outer gelcoat film, typically 0.5 to 0.8 mm thick. Gelcoats are designed to be durable, provide resistance against UV rays and hydrolysis.

Special gelcoats can be formulated to make the molds with which composite material parts will later be manufactured. These gelcoats require a very high level of durability to withstand the mechanical and thermal fatigue that occur during the pouring and curing process of the polymers.

The compositions of the resins vary widely depending on the function for which the gelcoat is to be used, but the most common are unsaturated polyester and epoxy. Within these two categories there are multiple subdivisions.

In addition to the pigment, a gelcoat may contain, if necessary, thixotropic additives to increase its toughness to vertical sections of the mold while curing occurs.

Gelcoats are formulated from polyester resins, and depending on the type of resin used, they can be mainly classified into two categories:

Orthophthalic Gelcoat: It is made with orthophthalic polyester resins, which stand out for their good rigidity and hardness. They are suitable for general applications in not too aggressive environments. Thanks to their resistance, they are commonly used as a base layer in painting systems, providing a firm and long-lasting support.

Isophthalic gelcoat: Formulated with isophthalic resins, this type offers greater resistance to water, chemicals and high temperatures. Due to these properties, isophthalic gelcoats are ideal for demanding environments, such as in the nautical sector, where they are used as a top coat on boat hulls. These are constantly exposed to humidity, salt and high temperatures, so more advanced protection is required.