Applying Rust Killer to under carriage /frame any reason not to ?

I have YET to see anything that claims to "kill" or "convert" rust work. They all just turn it black and it keeps right on the hell rustin. The only successful thing I've seen is to scrape the loose rust off, wire brush it down to bare metal where you can and coat it with something like POR15 or industrial oil based Rustoleum to seal it. Once rust is sealed from air, it cannot continue further.