I've used fiberglass repairs--very successfully--in rocker panels and floorboards. It helps prevent further rust, and I never had issues with them failing. I just used multiple layers of fiberglass cloth to build up strength.

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2010 Toyota matrix

'93 500 SEL
Well, I sold it. Just got to be too much to keep up with.
First time I haven't owned a MB since 1983.

'12 Volvo S80 T6
Needed something that wasn't as hard to deal with as my bad addiction

'18 Mazda Miata
No more boring cars for everyday transport!