Hey Brad,
My answer would be "all of those things" and "it depends". To be sure, if there's holes and missing metal, you will just cut it out and replace with new patches. But as you indicate you still have to deal with original (salvagable) metal that still might be pretty ugly with pits and rust. Inevitably, the original metal in these things is so old that there is no shiny metal left and a deep fine pitted oxidation layer is what you have to deal with. As for welding to original hazy rust metal, I always skim the edges to be welded with a 3 inch roloc 50 grit wheel (with right angle air grinder), and as long as I see some shiny metal there, welds come out fine.
As for prepping surfaces for paints, I believe you don't necessarily have to get the metal "white" (spotlessly clean metal) nowadays. I've used the air grinder with 3 inch roloc discs to get rid of the heavy old paint and rust, then an 5" electric "DA" sander with 80 grit discs to get all the rest of old paint off (and minor rust too). I sandblast nooks and crannies with a weenie sears sandblaster (particularly heavy pitting that is otherwise solid underneath). Finally sometimes you have to do some hand sanding in awkward areas.
The only time I use "converters" is actually on new metal. The stuff I used is phosphoric acid and I got it from Home Depot..it's a classic "metal Prep" stuff. It does etch the factory surface finish of the new metal and gives it "tooth". I wash the stuff off with lots of water and dry it quickly.
At this point all the old coatings are gone, and you should only see some patchy dark brown hazy coloring which I believe some call "tight" rust. For me, this is far enough. Master Series company (they sell industrial coatings) hint that having this "tight" rust haze is good because their coatings will stick very well to it. I don't know about the POR 15 stuff, I think that stuff is more of an anerobic type coating, and not easily manipulated for subsequent real paints.
Once I get to a tight rust phase, I coat with Master Series primer. When I'm ready for painting, I scuff sand the Master Series and then spray my epoxy primers. These work really well at sealing. If any filler work is necessary I put it on top of the epoxy. Once filler work is done, YOu can coat with epoxy again (to seal the fillers), then a "2K" smoothing primer, and finally color coats. These modern primers and paints will be orders of magnitude better than original, and if you marginally take care of the Jeep your resto work will outlast you.
Good Luck!
Dave
