I was also bothered by the surface rust, and deeper pitting rust on my rear subframe, so tackled it this weekend with a wire wheel, can of black rust paint, some surface prep, and Rock guard undercoating. My frame rails looked like yours, although not as heavily rusted, and I cleaned them up with a wire wheel. The body had some rust starting at joints and unions, which I also cleaned up and sprayed. You can see it by the grey and black. I just over-sprayed the grey with black pant and or undercoating. It is not a show vehicle, and I am trying to keep the weight down.
See finished photos.
You can see the heavier pitting near the bumper. That part was more rusted with most of the paint gone.
You may see rust still on the tow hitch and bolts. That is because I need to remove it and trim it for the larger spare. Also I removed the spare tire cable winch assembly, three 13 mm bolts, cleaned it up and painted it. It drops right out. I will lube it up and put it back.
Time: 2.5 hours
Cost $25 in material
Tools: Battery powered drill, wire wheel, paint, surface prep, rock guard undercoating,
PPE: Glasses, ear plugs, face mask for fumes and dust