I got rid of all the loose stuff and cut some of the thinner bits off. I cleaned all the rust I could off and coated it with POR. Cleaning the undercoating off was worse then the rust. Used a tin paint thinner can and screwed it in place on the inside. On the underside I coated the hole with fiberglass and used some PL Polyurethane on the inside. The nice coating of red started to flake off the PL Poly and with 20-20 hindsight I should have used fiberglass there too. The underside got a matching finish with 3M undercoating.

You can by all means fit or cover the hole with sheet metal and weld it on but to have a body shop repair a non-visible area may be overkill unless the car is show room like. A muffler shop has good thin-metal-welders for non-visible areas and you can finish it off. I was lucky that the mud flap mounting was not compromised.

Tom

Sleeping Rust, Shhh!