Some day I`ll fix that on the Tahoe, for now I`m just living with it. Been surface-rusted there since I got the thing all those years ago...
The REAL ANSWER is, IMO, to have it professionally repainted just like any other area. Ideally, then PPF.
For a quick-`n`-dirty fix that will at least keep the rust at bay, I`d do this:
-clean the area with a good prep solvent (I use Eastwood`s Pre)
-kill the rust, either by mechanical means (filament pen, sanding, diamond burrs on a rotary tool, etc.) or chemically, or both
-treat rusted area (I use Rust Bullet`s Metal Blast)
-prime with a paint-over-rust product (the only two that have pleased me 100% are Rust Bullet and, especially, KBS Coatings` Rust Seal, though Eastwood`s Rust Encapsulator is at least a *LOT* better than POR15, which I consider, uhm...unsatisfactory). Use multiple coats
At that point the rust will be taken care of and/but you`ll be left with a really obvious repaired area. *I* would (?would? ?will? man, I really oughta do this some time...) probably then take it to my body/paint guys and have them prime/spot-in the area. Somebody with greater confidence in their abilities might DIY the priming/painting
If yours is anything like mine, it`s a relatively flat/simple-contours area and once that`s not really obvious. No seams/welds/areas where killing the rust will be tricky. Getting that kind of out-of-sight area ready for the paint guy oughta be pretty straightforward, but IME it really does pay to use the *BEST* stuff; guys buy something like POR15/etc. because "you always hear about that stuff" and then wonder why things didn`t go all that well....
Done right, the KBS Rust Seal will *NOT* get worn through. Maybe the new touchup paintwork on top of it will after a while, but you won`t get getting down to bare (soon to rust) metal again if you have a few layers of that stuff on there.