I did this a month ago before I knew better: sanded off the rust as much as I could, cleaned with IPA, sprayed Mar-hyde 3509 one-step rust converter, heat resistant to 200°F (https://www.3m.com/3M/en_LB/p/d/v000207848/). As the manufacturer claimed, remaining rust turned to black as part of the latex primer sealer. Not sure how long it's going to last as is. I still have an option to coat it with caliper paint. Should I?