Quick tips on stopping the rust on the cheap

30 years ago I used some products from DuPont, they were for cleaning and prepping metal. Like all auto paint products, expensive enough . Seems the Ospho works as well as any thing for me. I am no bodyman but I have done ths stuff on qite a few ars since mid 80's. I always like to ask some questions of REAL bodymen when I wold have the chance.
I have used those " rust killing" products that COAT the rust and supposed seal it off. ALL the body guys I have asked say DO not use this if you will paint that area. OSPHO is not this ytype product. IT does not coat, it kills the rust and leaves a slight phosphate.