1 part epoxy primer and more primer..
OK. here is my $0.02 worth. I have done many cars in pieces. However, I think you want to take it too far. I assume that is why you got a big red X 'Disagree'. I'll get into that in a minute. On my 69 Barracuda, I completely disassembled it. The doors, fenders, hood, decklid, nose piece and front lower gravel shield were the main big pieces to do (9 BIG PIECES INCLUDING THE BODY SHELL). There were, of course, lots of small parts to do like hinges, hood latched, etc., but here I am only talking about the big parts. You could certainly do the pieces one at a time, but you will be wasting a lot of time. For example, if you stripped the doors and the hood, and then sprayed them with epoxy and then did the rest of the steps, it wouldn't take much longer to spray the primers on three pieces than on one. Plus, there is less time spent on prepping and cleaning your painting area. Whether you are spraying in your garage or a paint booth, the area needs to be thoroughly cleaned. Do you want to do that 9 separate times or only 3 times to get those 9 big pieces done. There is also time spent on cleaning your spray gun. That same economy of scale extends to filler work, sanding the filler, block sanding primer, etc. That being said, If you really want to do one piece at a time, go for it. When you are completely done with one piece (a door for example), Wrap it up (old blankets or Harbor Freight moving blankets) and store it CAREFULLY. You will be SO mad if the ready to paint parts get chipped or scratched. The only place I will disagree with you is that you really need to do the body shell in one piece at one time. I have been doing body work and painting for over 40 years, and I would NOT EVEN try to break the body shell up into sections as you suggested. You CAN do the engine compartment separately; I did. But that is all the further I would break down the body shell. That all being said, however you do it, do each piece with the steps in post 31 above. There are no shortcuts. This next part is my opinion, but a lot of professionals will agree. If you are painting a non-metallic color, you can easily paint the parts separately and then assemble the car. I'd paint 4 pieces one day, 4 the next and then the body shell on the third day. If you are going to paint a metallic color, you really need to paint all jams, back sides of hood, doors, decklid, fender lips, etc, and then assemble the car and paint it in one big piece. If you try to paint metallic in separate pieces over several days, you will very likely see slight differences between panels when they are assembled. I could write a book. If you have any further questions, PM me. Here are a few pictures of how I did my car.