painting one piece at a time?

I personally make a sealer using the epoxy primer by adding reducer. I imagine using the topcoat over the high build is done by many that way also.
There are several good ways to skin a cat.
I might also recommend Southern Polyurethanes, great products and support.
I am by no means a pro body man but I have personally done 12-15 cars or so. I have used several good bodymen over the last few decades and continue to try to learn for anyone and everyone.