Depends on how bad the rust is. As Martin says, there are products to encapsulate it or change it's ability to spread. Those usually require the loose rust to be scraped/wire wheeled, etc.
If it is rot out rust, you must cut out whatever you have to cut out. That's what patch panels are for.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
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