At this point any "tap and fill" may risk fracturing or delamination of the filler on the panel now. The rear of your cab reminds me of the hood we repaired for John Vannatta, where we found so much filler on the outside that the center peak disappeared just over halfway back on the hood. We stripped all the filler and did some hammer and dolly work, some epoxy primer, more hammer and dolly work, etc. In the end we had minimal filler used and a very stable hood skin free of any oil cans.
https://www.allmetalshaping.com/showthread.php?t=19619I find that working out the majority of the dents while in bare metal, followed by epoxy primer and block sanding to show any remaining highs and lows, is much more effective and less an issue of damage to filler as you can still work the panel with epoxy primer on it with no fear of compromising your previous work. Unfortunately, any attempts at "cave and pave" typically pushes metal outward before it goes inward, so I fear you would only be compounding high spots elsewhere. Cab rear panels were a common area of abuse on working trucks, I think you would be better served to regroup and perform a more thorough metal bumping on the panel. If this were something I had to tackle, it would be stripped and do all the metal work until it was an oil can free panel and then epoxy, block, and continue to fix highs and lows as they show up better with the epoxy on the panel. I just feel you are limited to what you can do without causing damage to the filler there now, short of just adding more filler. This would have made a good dent repair seminar for Homecoming...
