j-N-m:
I wanted to do a write-up on it but I was just too damn busy and I just wanted to get it done. Mine were pretty bad. What at first looked like just a few quarter-size holes and bubbling paint turned into extensive and numerous fist-sized holes. If I had a bigger Montero budget I would have brought it to a body shop and had new metal welded in. That will have to wait. Instead, I got after it with a right angle grinder and some course grit discs until I had the worst of the rust removed. Then I carefully jammed galvanized chicken wire and stainless screen into the holes as filler (galvanized wire made for a sturdy backer, stainless for the bulk filler you can carefully pack in). While I know this is not a proper repair, it seems like a good idea so that water and air can still pass through. After the holes were filled in that way, I switched to Bondo. None of my rust extended up around the curve on the top side. It was 95% only visible from under the truck. Because of that I wasn't too concerned with appearance and only sanded it down to a 200 grit I think.........then primer, paint, and on top of that a strip of rubberized undercoating (you can't see any of it when looking at the side of the truck from ground level).
If it lasts 3-4 years I'll be happy. It was getting so bad that I had to do something. Looks pretty good.