Every time I talk with a body man I ask that age-old question, metal or plastic? I'd have to say about every professional body man that I have talked with claims that the Bondo will hold just as good as the lead if it's applied properly.
And the opposite applies as well. Do not overlook the fact that an acid flux is required to tin the area where the lead has to go and I've seen that acid eat it's way through paint. Any product improperly used will probably fail. You can screw up lead easier than you can screw up bondo.
Personally, I think the new fiberglass fillers, being waterproof, are probably your best bet. Traditional bondo is good as long as it's not exposed to moisture.
And I'm an old school guy too, over 60 and started tinkering on cars in the late 50s.
