My earlier post about the Fluid Film was because I thought the undercoating was done to the inside of the frame. The F-F should still be put in there no matter what.
@Jerry Bransford 's idea of pressure washing would probably do most of the outside. It will also take away the semi loose rust that looks like it may still be there in various places. If you don't have a pressure washer, $10 at a decent quarter car wash should do it. (HA! Quarter car wash! How friggin' old am I?)
While you do have some thick flakes of rust between the skid plate and frame, it's a good sign that the bolts holding it in place were able to be loosened without causing bigger problems. Loosening them again while doing the pressure wash will let you get whatever is left of those flakes out of there.
A couple suggestions to help that area for the future:
Sanding and painting both the frame and the skid plate where they touch.
Putting a nice coating of F-F on both before tightening it back up should be done before every winter and even summer if you hit the ocean beach. Do it even if the sanding and painting doesn't happen.
Smear some anti seize on the bolt threads to keep them from rusting in place. F-F will also stop that rusting but I don't know how it affects bolts in use. FWIW, I used F-F on a couple of mine and will see how they are next month when I loosen up my skid plate to do my yearly F-F dose for winter.