They use it to make boats too, and on top of a substrate that absorbs resin it makes for a nice tight bond. Resin has 0 strength by itself and on top of metal there is very little mechanical bond. There's nowhere for that boundary layer of resin to go to. Now if you were talking about the adhesives and epoxies that are used along with mechanical fastening of some means, either welding or riveting it would make more sense. An encapsulated fiberglass structure over a frame ok, but as a sealer on metal I don't see it.