problem with fiberglas is differential temperature response causing the bond to fail enough to allow corrosion to continue and eventually causing complete release - fairly soon unless you use the special resin designed to bond with [clean] metal
JB Weld could work, but better for pinhole patches is solder, using a good hot iron and plumbing solder/paste flux .... next best would be 'all-metal' body filler .... whatever you do, mechanically cleaning the area well to completely rid it of rust is mandatory, use a rust converter if you can afford some
Bill
