From ‘Fiberglass blisters and barrier coats’: Fiberglass boatbuilding

In order to understand the process of osmosis and fiberglass blistering, it is first necessary to understand how the conventional, production fiberglass boat is, or at least was, built. This process has improved over the years; however, with the exception of improved materials and shop practices, it has remained essentially unchanged for decades.

First invented by a Swedish scientist, J�ns Jacob Berzelius, in the mid-19th century, polyester resin, the primary component of fiberglass boat construction, was not used commercially until World War II. The Japanese invasion of Far Eastern territories cut off supplies of natural varnishes that were used for electrical insulation. Polyester resin, made from the then readily available raw materials of oil and coal, filled the gap. By the 1950s, combined with glass filament made by Owens Corning, fiberglass-reinforced polyester (or FRP, as it is known within the industry) boatbuilding was born.

Unlike most timber and alloy vessels, which rely on the initial construction of a grid or skeleton frame structure over which a skin is applied, FRP is essentially built from the outside in, using a female mold. In short, the mold provides the shape of the part, the hull in this case, in reverse. Initially, a thin layer of gelcoat is sprayed into the waxed mold (the wax aids in releasing the completed fiberglass part from the mold). After the gelcoat has cured partially, a layer of resin is applied, which achieves an all-too-important chemical bond with the gelcoat. A layer of fiberglass, usually in the form of a thin, nonstructural mat — also known as the skin coat, which prevents print-through, a telegraphing of successive weave patterns of structural glass laminates — is applied next, followed by more resin. Additional, thicker layers are applied to provide structural rigidity and so on. This is what�s known as a fiberglass laminate. Fore and aft stringers, bulkheads, or unified grid pans or liners may be installed, adding rigidity and facilitating installation of furniture and machinery.

Many other factors and techniques may be involved in the complex process of laminate lay-up, including the addition of core, vacuum bagging, resin infusion, post curing, etc. This is, however, the established process, and many boats continue to be built in exactly this manner. Additionally, timing is a critical component in any FRP lamination in order to achieve proper molecular bonding of resin layers. Miss the chemical window, and an extremely strong primary bond becomes a substantially weaker, and potentially more water absorbent, secondary bond. This is particularly so for the gelcoat-to-skin-coat bond.