Gelcoat

Gelcoat is a specialized exterior surface layer used on fiber-reinforced polymer parts, most commonly in marine and automotive composites. It provides a smooth, glossy finish, introduces color, and acts as a protective barrier against water, UV light, and weathering. Most gelcoats are based on unsaturated polyester resin or vinyl ester, with epoxy variants used for high-performance applications. The layer is pigmented to supply color and UV stability and is formulated to cure to a hard, abrasion-resistant skin that is thinner than the structural laminate beneath.

In production, gelcoat is applied to a mold or tooling surface as the first step before lamination. It can be sprayed or brushed and is cured with a peroxide catalyst (commonly methyl ethyl ketone peroxide, MEKP). A mold-release agent, often a wax, is used to facilitate removal from the mold; the release layer is typically removed or sanded before the reinforcing laminate is applied.

The outer gelcoat layer serves cosmetic and protective roles: it provides gloss and color, reduces water absorption, and shields the underlying laminate from UV exposure and environmental attack. For repairs, the surface can be sanded and buffed to restore gloss, and color-matching is important for cosmetic results.

Limitations include UV aging, chalking, cracking or crazing under flex or impact, and potential osmotic blistering if moisture breaches the gelcoat. Gelcoats may be re-coated or repaired by re-gelcoating; epoxy gelcoats offer improved chemical resistance and adhesion in some applications but at higher cost.