It's very good to stop the rust. Just don't miss the leaks. And don't look behind the curtain, you won't like what you see!
Been there and done that - cleaned and painted everything outside but including frame rails & axle plates, plus everything on first and last six feet of frame from hitch and bumper... Then did the whole shell-off, frame rebuild, repaint and new floor a couple of years later.
The old-technology OEM asphalt-based 'undercoat' refuses to let POR bond unless its truly to bare metal, it leaves an oily film that must be solvent washed off even with sand blasting. The spots on my frame where I got the first POR-15 to bond to resisted angle grinder discs, wire brushes, 3M discs - the rest of it was more like shrink wrap on a retail store boxed widget.
I tried to overcoat the good looking spars and interior of the frame rails with a rustoleum black spray paint? Mistake, that lit them up with bright orange blobs of rust from trapping moisture or something.
The only structural flaw with doing a touch-up with floor on is the fiberglass insulation left crushed between the floor and frame, sure you can razor cut it away and make it look like its removed but the rusty fiberglass still wicks water in four, six, eight feet if it ever has live water touch it.
Also - if selected pieces of steel are replaced you will find the insulation crush occupies 1/8 - 3/16" space, before welding anything back in it should be shimmed down from floor to keep you from being Joe Somewhen that welded it up with 'high spots' standing proud that prevent a new floor sheet from laying flat if/when the floor panels (w/o cutting & welding) are changed out..
Note the high-strength 1/4" steel floor bolt corroded to needle sharp nub.
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