Por 15, rust oleum, tractor implement paint, what to use on rusted metal

lots good info above. anywhere I can, I use BIG wire cup on right angle grinder to clean best I can. and to find metal I must replace. NOT talking about the exterior of car here. just floors, etc....I have had success ( as some other FABO members with acid product called OSPHO. BUT it can have reaction with some primers, and paints, so test before topcoating with anything. it will clean rusty metal and leave a phosphate coating somewhat. it will take awhile to dry and be "done"...
over the last decades, I have used all kinds of rust inhibitors, mainly on floors, with good success as long as I cleaned the metal ( no scale!). I have gone over this with everything from POR-15, all brands of rust inhibitors, to plain old oil based tractor paint, expoxy primers, etc. all with good success, I ALWAYS topcoat these floors with very durable enamel, and again, tractor paint has served me well... not cheap but $30-35 per gal is good buy for hoe well it works I feel...
on the exterior of the car, I have asked your question to more than a few GOOD bodymen, one in particular does many streetrods, all say sand metal to BARE metal and the go with self etching epoxy primers. none wanted to treat with any rust inhibitor..... maybe they have had trouble with that in past!????
I have used POPHO on very pitted metal, after i sanded it down to bare and pitted metal. the OSPHO has worked well for me as long as I let it dry completely, may take several days. again, epoxy primers vary by acid ( something_), and can react differently. I even called the co for info and all I got was a DUH!
I hate rust so I want to do what I can to at least slow it down! as they say, rust never sleeps!?? LOL hope this helps....