When repairing a fibreglass panel, examine cracks in the surface coating to help determine the cause of the damage. Fibreglass panels consist of a reinforcing core of woven glass threads coated in a layer of polymer resin, or gelcoat. Acting as a cosmetic and protective coating, and typically around 1/32 inches thick, the gelcoat provides no structure to the panel. Crazing occurs when strain cracks the gelcoat but leaves the fibreglass underneath undamaged.
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True crazing, often called "alligatoring," appears as a random pattern of fine cracks in the gelcoat. It occurs more often on older fibreglass panels, especially from the 1960s and 1970s, when thick applications of gelcoat were common. A thick gelcoat resists flexing, becoming increasingly brittle over time, and may crack when temperature fluctuations of hot and cold weather cause expansion and contraction of the panels. Affecting only the coating, true crazing is easier to repair than problems affecting the structure of the panel.
Gelcoat is less flexible than the inner fibreglass core. If the panel is installed without adequate supports attached, or if there are too few structural laminates in the core, then tensile or compressive strain bends the panel, cracking the outer coating fist. Stress cracks in the gelcoat caused by structural weakness appear as "hinge cracks," or parallel lines. Alligatoring patterns--which look similar to true crazing, but are larger in scale--often indicate a weak core of flexing and separating layers. Stiffen or reinforce the panel first before repairing the gelcoat.
During fabrication, incorrect curing of the gelcoat creates surface cracks that have a pattern similar to true crazing. If the coating cures too quickly, due to hot conditions during manufacturing or from adding too much catalyst, then the resin will cure incorrectly and shrink. This adds stress to the gelcoat, putting it in tension and making it more likely to crack in the future. Curing the resins slowly, either from low temperatures or too little catalyst, weakens the resins, causing cracks shortly after being removal from the mould.
Though not true crazing, but often confused with it, radial-shaped cracks in the gelcoat form anywhere a concentrated load strikes the fibreglass panel. Closer inspection of the pattern reveals the direction of the stress: a bull's eye pattern of concentric circles indicates frontal impact, with the size of the inner circle relating to the size of the object it was struck with. Starburst or "spider" cracks result from an impact on the reverse side of the panel, which flexes the gelcoat outward in tensile strain creating a distinct pattern of cracks.
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