Famed yacht designer L. Francis Herreshoff disparagingly called it "frozen snot." Early builders called it a wonder material, impermeable to water and maintenance free. However you feel about fiberglass, there is no question that the material was responsible for a revolution in boatbuilding that opened up recreational boating to the average person.

One major key to that revolution was the development of the polyester resins that are integral to fiberglass construction. Many different resins, also called polymers, have since been developed, extending the capabilities available to builders and do-it-yourselfers – and making the entire topic more complicated.

PAEPolyester, vinylester and epoxy resins can be found all over modern boats, from deck to hull. But it pays to understand resins, because they are integral to the hull, the deck and many interior and exterior parts on a modern vessel. In short, you are already sailing on them. A basic knowledge of how these resins work and how they are used can help when it comes time to order parts, enact repairs and make other important decisions about your boat.

The basic concepts start simply enough: The resin used in marine applications is unsaturated polyester resin, a thermosetting material that uses heat produced in a chemical process to begin curing. The resin is applied over reinforcing material, such as fiberglass cloth or mat, to form the ultra-strong structure we know as fiberglass.

But not all resins are created equal. For example, polyester resin is good for initial construction, where the lamination can take place in one process to take advantage of good chemical bonding. Vinylester resin is a good choice for use below the waterline to provide resistance to water and prevent osmotic blistering. Epoxy resins are the best adhesives and are ideal for repairs to existing fiberglass or making lightweight parts.

Confused yet? Don't feel bad – it took the industry decades to sort it all out.

POLYESTER RESINS

While plastics such as Bakelite were around before World War II, the war hastened the development of the resins required for successful fiberglass construction. Aluminum was in short supply and fiberglass reinforced plastic structures offered a sound alternative. But the process early on was ugly.

Early polyester resins required heated metal molds to produce parts. The heat of the mold promoted the cross linking of the long molecules in the polyester resin, turning the syrupy liquid resin and the fiberglass reinforcing material into tough, strong structures. But this cross linking process was hard to control at the time and resulted in many molds being permanently bonded together. The steam heated metal molds were also very expensive to manufacture, meaning only high-production parts could be considered.

PAUL ESTERLEPolyester and epoxy resins can be used to make repairs and create projects that are both strong and relatively light.

Two key developments were crucial to the explosive expansion of fiberglass products: catalysts, or hardners, and mold releases. The development of organic peroxide catalysts allowed the polyester resin to crosslink and cure at room temperatures in simple, unheated molds. The development of effective mold releases, such as carnauba wax and poly vinyl alcohol (PVA) allowed the molded parts to be reliably removed from their molds. A new industry was born.

Polyester resin became to mainstay of the budding fiberglass boatbuilding industry and sparked new boat builders in places like California and Florida. Many small companies took wooden hulls and made fiberglass molds from them. These backyard builders began cranking out boats of all descriptions.

Early fiberglass boat builders built by eye rather than by engineered design. They had the notion that if a little fiberglass was good more was even better. This meant that many early designs were overbuilt. Many of those early boats are still going strong and have been refinished. Other early boats have been neglected and sit in the back lots of boat yards, too shabby to refinish them and too strong to molder away like a wooden boat would. Disposal of old fiberglass hulls remains a problem, often solved with a chain saw and a trip to the landfill.