Ok, I talked to the owner of the garage where the FB will be fixed for traffic registration. It was clear to him that the rust is from the inside .... however he recommends cavity preservation and sealing the rust with creep oil, the latter needs to be refreshed once a year. If the rust increases despite these measures replacing the whole panel would be his prefered option in order to maintain an original appearance even regarding the back side of the sheet metal. Furthermore, only replacing a small piece of sheet metal would cause heat etc. that could harm existing sourounding sheet metal perhaps resulting in new rust. Summary: retain original sheet metal as long as possible. He is enthusiastic on the overal sheet metal condition having seldomly seen a similar F-body. To him my FB is a true survivor - despite one (older) repaint. No fixes to hide questionale areas etc. My FB seems to be a good remedy for my perfectionism :-)
