I don't know a whole lot about beating dents out or pulling racked cars back into shape, but I can shape bondo. 13 years in a 'glass shop doing a fair bit of tooling as well as sanding filler on one off shapes taught me a few things. I didn't grow up doing it, so starting at 24, it was not the easiest thing for me to learn.
Skim the whole area, don't try to just do small sections.
If I want to save time on big panels, I always use the biggest spreader that I can fit on the panel, even if it is a 4' aluminum straightedge, laid over to get it to curve. More than once, I have mixed up an entire gallon of filler and spread it like this. If you are tooling up for a 5 x 6 tonneau cover or a roof or a hood this gallon won't go far.
Yes to the "bondo hog"
Yes to shaping with 36 or 40 on a long board.
Yes to spraying polyester "filled" primer when you get it close.. I use stuff that can build .040"..that stuff makes a huge difference.
Get a can of guide coat black and use it. Even if you are still knocking it down with 40 or 80, on a big panel it will make a difference.
Close your eyes and feel the panel. a light touch moving somewhat fast will find alot of highs and lows.. The hard part is knowing how to interpret what you feel.
I like doing some of my more agressive shaping with a sureform file, or "cheese grater"..the less time you spend shaping a big job the less chance you will sand off stuff you want to keep.
Yes to making the bondo more spreadable, most all of the BPO catalyzed fillers are compatable with each other.
I would not use regular polyester laminating resin though, as we did 20 years ago. (it is not double promoted, and really doesn't catalyze properly with just BPO.)As Brian said, there are much better additives. I use the evercoat 400 as a spot putty and bondo thinner myself.
Use the biggest block that you can fit on the panel. I have a 3' x 8" wide board for big stuff. I use paper from a belt sander for paper, it lasts FOREVER!.
I know that Brian has said some of this, and some of what works for me is not the way others do it.
I don't dance either.