anti-rust? [Update]

Anyone know of a reputable spray that I can shoot into a fender cavity to slow the progress of rust? I have a little rust from a repair from an accident early in the life of the vehicle and the shop I took it to get an estimate says its coming from between panels. He suggested spraying into the cavity to slow it down. any good products to try? I know it’s not a cure but it gives me more time to figure out what I want to do with it.

UPDATE - Turns out I forgot that I already bought fluid film to protect my hilift. So I sprayed the area with that.  boom.