no primer, just cleaned them off, little bit of sanding, and sprayed paint.
Listen. No matter how good of a job you do, the rust WILL come back. I did my mp3 extremely well, took my time did everything by the book. Took my 8 hours one day to do 5 coats on 4 calipers with G2 caliper paint that costed me about 50 US bucks. Lasted 1 yr, and the rust came back.
Not doing that no more. few bucks a can for high temp paint. 2 hours, done. Freshen it up every spring with a light coat. I did a color that's close to stock, so it was much easier to do. Changing the color would involve a little more work, and more care. If I scratch my silver calipers now, or if they peel, guess what, it's silver under neath. No big deal.