Let me ask what type of fiberglass trailer you are inquiring a about, different fiberglass designs have different types of finish on them, though most have gelcoat, if it is gelcoated, you may not need to paint, you may just need some polishing, gelcoat is much thicker than paint, so even when heavily oxidized it can often be restored to near new looks with some work using a double action polisher, and a light polishing compound and wax