I have been restoring old cars for almost 40 years. I follow the FAA specification for steel and iron parts in airplanes. I have also done aging studies over years with cars left out in the whether. Here is the process 1. Remove rust to bare metal as much as possible. high speed wire brushing is the easiest for most people. Sand blasting is best. 2. Treat metal with phosphoric acid metal prep. let dry 24 hrs. 3. Paint metal with "yellow death" from Dupont. (This is absolutely critical.) Let dry /cure at least 48 hours. 4. Cover with epoxy chromate primer. 5. Cover with sandable primer 3 coats minimum. Let dry sand with 400 grit. 6. Paint with finish paint. Dr. Bob