For years, farmers and antique car owners have used nothing more than motor oil, squirted into the body cavities and frame members where water gathers. Stay on top of nicks and surface rust. Oil shuts O2 off completely eliminating any further rust. In exposed areas frequent rinsing is the key.
Had 2 86 Novas in rust belt, that I treated every two years for 7 (4 treatments) years, turned one over to my son for the next 8. He never treated it and it never rusted (Novas were bad too, rockers and rear quarters). Traded it in at 280,000 miles with a very faded paint job, a bunch of dents (he never waxed it) but rust free.
Unibody is easiest to do. Floor rust is usually caused by water and salt you track into car and rusts from the inside. So coating from outside is only partly helpful. Peel back carpet and coat metal from “INSIDE”…cover with poly, relay carpet. There are commercially available oil based rust inhibitors. I used grease on my trucks and old 78 Subaru in prime rust areas. Make sure your car has a water proof backing on the carpet…some were “indoor” and actually accelerated the rust.
Average treatment time for uni. is about 10 min. from set up once you know the car. You want to spray into any drain hole water drains from till it runs out adjacent. Other particulars to minimize drain mess, but talk to a few mechanics (old ones) who can get you started…check antique car web sites as well. If you use more than a quart of motor oil from a dedicated garden sprayer…your overdoing it.
Doing this helps you really get to know your car and how some (esp. older Subarus and others) were designed to rust. Who would ever use an open cell foam gasket to mount fenders if they didn’t, and drain holes higher than lowest points…etc. A joke.
Some here will tout that newer cars are “rust free”. Not from my vantage point under cars, many are just better at hiding their engineering intent… designed to make you trade earlier than you should.
I’ve rambled too long, but have tons of stories how we have “frozen” rust on friends cars and trucks and milked them for years more through inspections, safely until the mechanics gave out first.