I'm not a body/fender guy but I've played with glass and bondo enough to know it never acts quite like you want it to. My thinking on this particular repair is that if you do a good job pulling the dent the surface of the metal is going to come of fairly flat (uniform). Yes, you can put a layer of glass & resin over the pull hole, but when you sand everything down to the level of the rest of the metal, you will have sanded off nearly all of the glass cloth - thereby defeating your purpose and possibly even sanding right through to expose the hole again.
The best fix, obviously, is to weld up the hole and grind it down. The next best (IMHO) is to use the normal shade tree approach, which is to press the bondo into the hole allowing it to ooze through the whole and then expand a bit on the back side making somewhat of a "plug". Again, any fill of a hole of gap with bondo is NOT going to give you the best results. But if you are just doing a quick fix, it will last for a little while.
Always learning...and sharing what I've learned.
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