My Minnesota bus has some rust on the bottom, nothing too bad except one wheel well that needed some new sheet metal. To try and slow the development of worse rust I'm trying this....
I first power washed and waited a few days for dry weather. Put some effort into a carbide scrapper against the rusty places like the frame rails to cut off rust and smooth the areas down a bit. Next I used mineral spirits to thin down the product I bought called Cosmoline Black Rust Veto 344. Next step, rolled and brushed on the product. It's pretty easy to apply, about 1/3 done after 6 hours of labor.
Lastly I used a lot of GoJo and a 3M scratch pad to wash my arms.
Hope I have given the rust a little nap, it looks much better black than orange under the bus. Long term.... just have to see how it hangs in there.