Duraglass is a fiberglass filler that doesn't absorb water. I use it on keels that sit in the water for 6-12 months and it does fine. Corvette resto shops use it a lot.

If you can't weld I think if you grind, then POR/paint the rest, and fill with Duraglass you will have a decent repair.

Bondo is fine if you use it for what it is ment for. Filling a hole like that is not what bondo is for. Bondo is for smoothing out small inperfections on the cars surface before painting.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200