I used fiberglass fabric with POR-Putty instead of resin on several bad rust spots on the theory that it would be more flexible and less prone to cracking. It was a few years a go but I seem to remember that the problem with using glass on metal is that they flex differently and the glass will crack and let in moisture leading to more rust. The POR-Putty is supposed to avoid that. And it has so far on my truck.
Frankly I'm not sure I worry about your guy using glass. Once the paint's on, it will look great and you'll just have to watch it for signs of rust. If your truck's like mine, there'll be lots of other things more pressing to worry about.
Good luck!
Wilber
