Yeah i live in the rust belt myself. My bodyshop guy tells me there are chemicals out there to hold it. Where the eye does not see it. I might consider using a rust converter. (changes rust to iron oxide) Then take a small airbrush or paintbrush and paint area after prepping properly. Something like dremel drill with small wire brush first to remove all loose material. Basically you cannot stop rust only slow it down unless you either cut it out or convert it. Surface rust on fenders originating from trim clip holes and spreading out under the paint seem to be a no cost option on some earlier mercedes bodys. When they reach an inch in diameter I just take off chrome, sandblast with very small recirculating blaster,metal prep and refinish small area with medium airbrush. Before starting but after removing trim i remove offending plastic trim clip retainer to properly treat plastic retainer hole as thats where the problem originated i believe. Does not exactly match but at least stops the rust starting up again. The bubbly paint appearance is gone as well. I have to do something as the spread rate seems pretty fast and if left will hole body in that area in a couple of years or so. Moisture behind paint seems to increase rate of rust spread. Yes you can treat a bodys inner panels and structual components to slow some of it down. In my opinion you almost have to do it yourself though as it takes quite a bit of time to do a though job. But as it is labour intensive it is also cheap.