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In fiberglass construction there are two types of bonding, primary and secondary. In primary bonding the resin that bonds the layers of glass together is cross linked (resin molecules intertwine) to form a continuous structure. Primary bonds can only be made if the additional layers of glass/resin are added before the previous layer has cured. Layers added after curing are attached by secondary bonds. They are simply glued to the previous layer with no cross linking of the resin molecules. The strength of the secondary bond relies on the adhesion of the resin to the underlying surface. Both polyester and epoxy resins are good in primary bonding. Epoxy is superior to polyester in secondary bonding.
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When a boat is laid up the entire hull is made in a continuous process resulting in primary bonding throughout. When components and structures are added post cure, the bonding of these units to the hull is secondary. Components that are installed with secondary bonding include engine beds, floors (transverse stringers), stiffeners, knees and bulkheads. The hull/deck joint is also a secondary bond. When making repairs to a fiberglass structure the bonds to the old fiberglass are secondary. Epoxy is better for these repairs because of it's superior strength in secondary bonds.
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Tabbing is used to add strength to secondary bonded, structural members like bulkheads, floors and interior fixtures of wood or fiberglass. Epoxy with filler can be used to form a fillet where structureal members or fixtures meet (typcally at right angles). Glass tape applied before the fillet cures will form a primary bond to the fillet and a strong seconary bond where it laps onto both meeting surfaces. Biaxial tape or fabric yields the highest strength in these highly stressed applications.
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