I sanded and painted my freezer with rustoleum. 2 coats. The rust still came back (it was rusty when I got it) and is eating through the paint. I keep it outside. Got it about 9 months ago.
I sanded the crap out of it with an orbital.
The paint is not flaking at all. I thought that the rustoleum was all the rust protection that I needed, but I was wrong. So, from my experience, I would spend an extra $5 or $10 and prime with a rust inhibitor. Then paint.