It depends on how much you want to spend. I have had good luck with POR-15, which is "Paint Over Rust". You don't have to get to bare metal or prime, you just have to get to an intact (not scaling) surface. It cures by exposure to moisture, and it gets very hard. There are a few drawbacks. It's expensive. It's messy to work with- do not get it on your hands, it is tough to get off. Because it's moisture cured, once you open the care you're committed. The instructions say it should be top coated for UV protection.
I had the repair shop paint the loader arms on my CTL with it after they rewelded them. My CTL worked in the cement business before I bought it, and that tore up the steel. The shop that rewelded the arms didn't have the facilities to do much finish work, but I did get them to rough up the rusty steel and coat it with POR. They used a spray can for the top coat. It looked pretty good when they were finished with it, and it has held up to heavy use for a year now.