Gelcoat is a material used to provide a high-quality finish on the visible surface of a fibre-reinforced composite material

Composite material

Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

. The most common gelcoats are based on epoxy

Epoxy

Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

 or unsaturated polyester

Polyester

Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...

 resin

Resin

Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

 chemistry. Gelcoats are modified resins which are applied to moulds in the liquid state. They are cured to form crosslinked polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s and are subsequently backed with composite polymer matrices, often mixtures of polyester resin and fiberglass

Fiberglass

Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

 or epoxy resin with glass.

The manufactured component, when sufficiently cured

Curing (chemistry)

Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...

 and removed from the mold, presents the gelcoated surface. This is usually pigment

Pigment

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

ed to provide a coloured, glossy surface which improves the aesthetic appearance of the article, such as a counter made with cultured marble.

Many marine craft and aircraft are manufactured using composite materials with an outer layer of gelcoat, typically 0.5 mm to 0.8 mm (0.02 in to 0.03 in) thick. Gelcoats are designed to be durable, providing resistance to ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 degradation and hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

.

Specialized gelcoats can be used to manufacture the moulds which in turn are used to manufacture components. These require very high levels of durability to overcome the mechanical and thermal stresses

Stress (physics)

In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

 encountered during the curing and demoulding processes.

Suitable resin chemistries for the manufacture of gelcoats vary, but the most commonly encountered are unsaturated

Saturation (chemistry)

In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...

 polyesters or epoxies. Within each of these categories, the resin chemistries are further subdivided.

In addition to any pigment a gelcoat will, if necessary, contain a thixotropic

Thixotropy

Thixotropy is the property of certain gels or fluids that are thick under normal conditions, but flow over time when shaken, agitated, or otherwise stressed...

additive to assist its tenacity to vertical portions of the mould whilst it cures.

Repairs

It is possible to repair scratches, chips, cracks and worn areas of gelcoat fairly easily. Firstly, any loose parts should be chipped away, and then the area to be repaired should be abraded with sandpaper (150-220 grit). Next, topcoat should be applied to the area after mixing with catalyst to start the curing process. Once cured, the topcoat is sanded down to blend it with the original gelcoat, and then polished to restore its shine.