Yes, for the under fenders, I am thinking of POR 15 or rust bullet in flat black brushed on with the wheel off. (After a nice wirebrush on the end of the drill.)
I thought about this when I decided to paint, and at the time, since I didn't have a whole lot of room, leaving the fenders/aprons/bibs hanging on the truck made sense. A complete removal / sandblast / would be idea.
It has some black undercoating under the tub that is in good shape. The line of light rust you see under the fender is the undercoating worn away from many gravel roads.
I'm also going to put some wax on it.