Well, surface rust is surface rust; it will never go away without proper prep. If you just want it clean looking, pressure wash, wire brush as much as possible, rustoleum flat black. It will always have that sort of brownish black color after a couple of days but it will be clean and it will retard the rust to a certain extent, but you will need to keep after it.
Don't forget that the frame is a c- section with the open portion to the inside. Mud will pack up,in there and it is hard to see or get to from the outside of the vehicle, ie. pressure washing form the wheel wells, you need to be spraying from the center line toward the outside, that's why having it on a rack is best, but rarely possible.