**Update**
The sanding process is A LOT harder than I thought it would be. The oil undercoat did not come off from sanding. With closer inspection I noticed it just spread thinner.
I ended up having to wipe most of the frame, underside of bed, and inside of fenders with mineral spirits. That softened the coating and I was able to get it off with a sharp scrapper and wire brush. I also used a ton of shop rags, foam brushes, and mineral spirits.
After the coating was removed I went over rust with a needle scaler, wire wheel, poly cup brush, 80 grit sandpaper, then 120 grit. I think 120 is as high as I will go before primer. I am tired of sanding the underside of the truck and want to move to the body metal.
My only major concern is that the paint (original, not CARC) is flaking patchy under the fender. It is very strange compared to other areas of the truck. 60/80 grit sandpaper and wire/poly wheels don't really remove it, but it comes right off with a needle scaler, however it damages the thin metal. When I blow high pressure air at it, it flakes in random areas. I can also peel it with a scraper and a lot of pressure. In some ways it seems to unstable to paint over, but in other ways it is a pain in the a$$ to get off. Advice? I have been working on the fenders for many hours and might just call it done and prime.
As for work on the body sheet metal, I have wire wheeled all the rust spots, and now need to feather them out with 80 and 120 grit paper.
I have some questions. On the body should I just sand to 120, then prime, and paint over the primer? Should I sand to something like 320, prime, then paint? Or should I sand to 120, then prime, and sand the primer to 320 or 400 before painting? I am using rustoleum rusty metal primer and alkyd enamel paint.