FRP is frequently used in highly corrosive environments as it is inherently corrosion resistant and does not rust like steel. So why cover it with a protective coating?  One of the primary reasons is to provide protection from ultraviolet (UV) light. Exposure to UV degrades resin at the molecular level by breaking the carbon chains of the polymer.  An analogy for the degradation is the chalking of epoxy coatings when exposed to sunlight.  In the case of FRP, as the breakdown of the outer surface continues over time, glass fibers are exposed and the mechanical properties of the laminate are reduced. If the surface is already degraded from UV, coatings can be applied to stop further deterioration.