Fiberglass entry doors obviously use fiberglass as an external material. However, the outer fiberglass overlays a support system of wood rails and stiles in addition to filler polyurethane foam insulation. This may affect the capacity of any fiberglass door to resist harsh sun damage.
The relative quality of a fiberglass entry door has an effect on its resistance to damage, including damage from harsh sun. Low-quality fiberglass may fall apart or crack and allow paint to bubble from water leaking into the door as a result. Such a circumstances increase a door's susceptibility to any form of weather damage.
A high-quality fiberglass door is much more likely to resist damage from harsh sun. Such a quality fiberglass door may exhibit no deterioration in its original appearance a few years after installation.