Our buddy Spraytech will be able to give us the full scoop, but in brief, flex agents are intended for use on pliable plastics, such as contemporary bumpers. There are some paints that have enough innate flexibility that additives aren't necessary. If you use a flex additive, you need to follow the manufacturer's mix ratios precisely as this is one of those things where "more is better" is a definite no-no. I don't believe there would be any advantage to using it on a fiberglass piece as glass doesn't flex as much as the various plastics do. The paint companies I used to do business indicated that the film strenghth of the paint is diminished somewhat by the additive (that's why it's an additive rather than just included in the paint).