Warping
One notable feature of fiberglass is that the resins used are subject to contraction during the curing process. For polyester this contraction is often of the order of 5-6%, and for epoxy it can be much lower, about 2%.
When formed as part of fiberglass, because the fibers don't contract, the differential can create changes in the shape of the part during cure. Distortions will usually appear hours, days or weeks after the resin has set.
While this can be minimised by symmetric use of the fibers in the design, nevertheless internal stresses are created, and if these become too great, then cracks will form.
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