Originally Posted by 50bowTie
They do convert the rust to "something".
You CAN paint this "something".
But there is a good chance that someday sooner or later that acid will creep through your paint and do nasty things. Why risk it? If you can abrade the metal until it's shinny clean, and cut out the cancer spots/weld in patches,...
An Epoxy will last many more years than any "Rust Converter" could ever dream of.
The "something" is iron phosphate. Rust is "iron oxide"... an ongoing process. I imagine all of these products (like Phospho) contain phosphoric acid... which converts the iron oxide to iron phosphate. It is dead, and just lays there forever. The acid is not a problem if you can follow the directions on the can. Another trick is to wipe it with degreaser to help neutralize any residual acid.
I will add that right now I am completely redoing an AMX, which was stripped to bare metal and then epoxied. It had been a number of years, and when I sanded through the top coats, and into the primer, I found little buds of rust growing every where. Further investigation show that it was rust under all of the bodywork too!
We took it all off, and started over. A simple treatment with the antique "metalprep" (which contains phosphoric acid) would have saved the guy at least $5000. I have used it for about 40 years (hundreds of paint jobs) without a single failure. You will hear lots of new ideas and technology... but I like simple, inexpensive solutions that work.
I would recommend any phosphoric acid product that performs this simple conversion process. It has been proven for longer than most of the painters on here have been painting.
Just remember... rust starts on a microscopic level, which you can't see. Play it safe, and add some cheap insurance.