With all due respect to the other opinions and perspectives provided in this thread, this statement you've made is, based on my personal experience, the case.
Provided you remove loose paint and rust, and all oils and grease and your surface is fully dry, you can simply brush apply the Rustoleum with no primer.
The advantage of brush applying is that you can drive the coating into the toothed and rough metal surface imperfections which will displace air and moisture that spray painting can't address.
... But, take whatever steps you feel most comfortable with. I'm just saying that based on the process I used and results I achieved, priming is not necessary provided you use the brush method.
Started pulling the bed (I say "started" because I ran out of gas Saturday grinding on the bed bolts that wanted to spin in place), so I've officially started this journey. Hoping I'm actually to the point of grinding and painting by this weekend....at least on the front frame horns since that will have to be completed before I can do the steering gear and core support.
We've discussed application (brushing) and primer (not critical)...what about layering? How did you handle putting down multiple coats? How many did you do? Were the first ones kind of light or kind of heavy? Did you wait for each coat to fully dry or did you put the next coat on while the last one was still a bit tacky?
Appreciate any further advice you can give. I'm not getting my RO Hammered free anymore (new job), but I still think that's the route I want to go.