Jody I have multiple go to's depending on the size of the part,acceseabilty,the finish I'm after,and how much time and trouble I want to spend. Sandblasting no need to explain. Electrolysis
@Dryriver1 and
@Turbo4whl have already described. I'll use ospho in hard to access or to protect panels I've taken to metal or rust I'm just trying to stop. If you want to use polyurethane over follow the directions at SPI polyurethane. It's the only acid wash they recommend and in my opinion the magic as to why paint over rust products work Acid Wash
https://www.ospho.com/directions.htm . Another that works real well at removing rust and leaving clean metal on big things is get it wet with a solution of citric acid and water wet some rags wetted with the same solution and throw on the part to keep it wet. I'll try to find you a write up. Citric acid is what I'd use on a roof in a bed etc. Phosphoric acid inside doors and wheel wells, trunks back sides of panels etc.electrolysis on smaller stuff. Couldn't find a great write up so here's a crappy write up but you'll get the basics. Use 10 percent citric acid more or less.https://www-instructables-com.cdn.ampproject.org/v/s/www.instructables.com/Removing-Rust-with-Citric-Acid/?amp_gsa=1&_js_v=a9&_page=true&usqp=mq331AQIUAKwASCAAgM%3D#amp_tf=From%20%251%24s&aoh=16937921576733&referrer=https%3A%2F%2Fwww.google.com&share=https%3A%2F%2Fwww.instructables.com%2FRemoving-Rust-with-Citric-Acid%2F
One more trick attach a wire brush to a worn out sawzall blade to remove rust.