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.

As the industry began to mature, resin developments kept pace. Resins formulated to provide rich colors were developed. Sprayed into a mold, these gel coats formed the impressively shiny hulls we now expect on fiberglass boats. As spray equipment improved, gel coats were applied in several colors to provide molded-in cove and boot stripes.

WEST MARINEPolyester resin comes in two forms, laminating and finishing resin.Indeed, polyester resins don't cure to a hard surface in the presence of air. Rather, the surface remains tacky. This air-inhibited quality works well when subsequent layers of fiberglass are applied in the mold, because each new layer bonds well to the lower layer. The final application of resin contains a wax that rises to the surface of the fiberglass and seals it from the air, allowing it to cure to a hard surface. It is for this reason that the polyester resin found in marine stores comes in two forms, laminating (wax free) and finishing (with wax).

VINYLESTER RESIN

Fiberglass boatbuilding continued apace until the 60's and 70's, when increases in oil prices caused many polyester resin suppliers to experiment with new formulations. These proved to be more permeable to water than earlier resins and were in some cases susceptible to osmotic blistering under the gel coat and between the inner laminations of the hull itself. Water would migrate behind the gel coat and react with the resin to form blisters of varying sizes. The blisters can range from small dime-sized problems to serious hull de-lamination.

As a result, new resin formulations were developed. One of them is vinylester resin, a product with a similar chemical makeup to polyester resin, but with some significant enhancements. The resulting resin is stronger and less likely to form blisters than polyester, but less expensive than epoxy resins.

Vinylester resin is often used below the waterline in boats to better resist blistering (many hulls laminated with this resin even carry warrantees). Vinylester resin, like polyester, doesn't form strong secondary bonds to cured surfaces, so it is best suited for initial construction.

Both polyester and vinylester resins are cured with the use of a catalyst, which starts the process but does not take part in the chemical reaction. The curing process is exothermic, meaning it generates heat as the resin cures. If too much resin is catalyzed, the mixture can heat up to the point where the resin boils, or even catches fire. It is for that reason that these materials should be treated with respect; all work should be done with proper ventilation, eye protection, nitrile gloves and other precautions.

WEST MARINEEpoxy resin is a strong adhesive, good for repairs.But the process does give the builder flexibility. The curing time, or "pot life," of the resin can be adjusted for temperature and humidity by varying the amount of catalyst added. Generally speaking, the warmer the weather the less catalyst required. The resins sold in marine stores usually have a catalyst chart on the container specifying the amount of catalyst required at various temperatures.

EPOXY RESINS

A third class of resins is the epoxies. Epoxies differ from their polyester and vinylester cousins in that a hardener is used instead of a catalyst. The hardener actually ends up as part of the cured epoxy.

Epoxies offer superior strength, toughness and resistance to water, but their higher costs relegate their use to high performance power and sail boats. For repair work to existing fiberglass boat, however, epoxy is the product of choice. That is because epoxy provides a much stronger secondary bond to cured fiberglass than either polyester or vinylester resins.

Because the hardener becomes part of the cured epoxy, the ratio of resin to hardener is fixed. Additional hardener doesn't make the epoxy cure any faster (in fact, it results in weaker epoxy because the excess hardener remains in the fiberglass laminate). Most epoxy suppliers offer different hardeners with different cure times for cooler or warmer weather.

The ratio of resin to hardener differs too. One to one, 2:1, 5:1 and 7:1 ratios are common. Many epoxy suppliers offer pumps for their epoxy containers. One stroke of each pump automatically provides the right ratio.

There are also a host of additives available to modify epoxy to specific jobs. Among them are colloidal silica, which thickens the epoxy and helps prevent sags and runs; high-density filler, which provides additional strength; low-density filler, which transforms epoxy into a lightweight and easily-sanded material for filling holes and fairing surfaces; barrier coat additive, which enhances epoxy's already high resistance to water; and pigments to adjust color.

These additives can be added to the resin/hardener mixture to modify the characteristics of the base epoxy resin to suit specific application. For example, high-density filler and colloidal silica can be added to epoxy to form a strong adhesive that won't sag or flow out of a joint. Low-density filler, along with a small amount of colloidal silica, forms an easily-sanded filler that can be used to smooth a surface for painting.

Epoxy can also be used to barrier coat a hull that was laminated using polyester resin. This is often done after a polyester hull has developed blisters and has had them repaired. To prevent further blistering, the underwater portion of the hull is coated with epoxy to form a more impermeable barrier. Often six or more coats of epoxy are required to build up a sufficiently thick layer of epoxy to halt water migration into the hull and prevent blistering.

Epoxy does have one bad limitation: it has very little resistance to UV rays. Exposed to sunlight, epoxy will turn brown and degrade over time. For this reason, epoxy-coated surfaces should be protected with paint or another protective coating.

For those interested in learning more about resins, there are a shelf full of books. Information can also be found on manufacturer websites, from talking to pros and, of course, the old-fashioned way – rolling up your sleeves.

Paul Esterle is Managing Editor of Nor'easter Magazine and Technical Editor for Small Craft Advisor Magazine, and his freelance work has appeared in Sail, BoatWorks, Voyaging and Good Old Boat magazines. He has produced a series of boating videos and lectures widely.