I finished masking the body, and sprayed a coat of epoxy primer on several spots where I had sanded all the way through the primer-surfacer (or even to the metal). As soon as that had dried, I sprayed a couple of coats of primer-surfacer everywhere.
Then I started wet-sanding the entire body. Wet-sanding is not at all unpleasant. It does not involve nasty chemicals, foul odors, loud noises, heavy lifting, special clothing, dust, etc. It’s just somewhat boring, and you tend to slowly sand the skin off of your fingertips. You also have to be careful not to overdo it and sand all the way through the primer. I am about halfway finished. Hopefully I will be done in the 3 or 4 days.
Once the wet-sanding is finished, I need to decide how to deal with the aluminum molding strips that go around the edge of the roof fabric. I can install them before painting the body, or paint them separately and install them afterwards. There are advantages and disadvantages to either approach. In any case, I first need to finish preparing the moldings: cutting each piece to the correct length, drilling nail holes, sanding them, and priming them. I have ordered some special zinc chromate aluminum primer, and some butyl rubber caulk that was recommended for sealing the molding.