Lately, I've been experimenting with aerosol spray white grease on the rusted areas of one of my decaying classics. In the past I've had good luck stoppng or slowing rust by pouring/squirting 90 weight gear oil into enclosed areas, such as rocker panels and the insides of the rear wheel arches, that would otherwise require major cutting and welding. The stuff does stink though. There are also some commercial rust-stoppers availible.
Try a search. There have been other rust treatments discussed on this forum in the past.

Happy Motoring, Mark

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