The best way to stop rust is to remove it. Cover the whole car with a heavy tarp. Cut the center out around the rust and tape it down with duct tape. Get a small sand blaster and sand blast the rust off. Then paint it use a good epoxy primer . After that scuff it and primer it with a fill in sand two part primer . Block sand it smooth and paint it. The rust will be gone for ever.
The most important is to cover and tape up where ever you don't want the sand.
I found all those products they sell to stop rust do not last as a permanent fix
We blast all surface rust off and then strip the rest of the carcar with paint stripper. after taping all the gaps. Then sand the bare metal with 220. We never paint over old paint. You can see the grinder marks on the body under the original paint done at the manufacture, You actually see what you are starting with. If you sand the car you never know whats under the paint.
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^^^@Oldmanmopar That is the ultimate method IF you have a guy that can Do it! Worping metal is easy , be careful.
Second best. You have pitted metal I bet. It can be DA, use a big wire CUP on a right angle grinder, but that does not take care of pitted metal with rust in that pit. I use a phosphoric acid solution to attack that. There are many hundreds of threads here on that. I personally, have not used a "rust convertor" type product in 40 odd years, every since every body man I had talked to said don't. Never needed it.
Like said above^^^ after the metal is clean, using a top grade epoxy primer is keep, 3 coats! Then TOPCOAT.