Upon reflection, I will say that I use a good non sanding sealer after I'm completely through with primer. The reason is that a lot of color coats will cause primer to "swell", revealing even the most minute flaws (sand scratches, etc), and then it shrinks unevenly.
If you really want to avoid such problems, let your primer/surfacer set for 2-4 weeks after you finish all sanding, then seal it, then let it set for at least a week. Then you'll have the absolute minimum penetration of any solvents in the top coat.
I learned most of what I know from working with my Dad. He was doing paint and body work in the fifties, and was even really good at doing lead. He was one of the last 2-3 people within a couple hundred miles who could do lead work. He never could teach me to do it reasonably well. I could probably do it now, as I have more patience. But I learned the trade out of necessity 30 years ago. I simply could not afford to pay anyone. But I learned the old school way. In fact, I first started fooling with it at the age of 6, when Dad was restoring a 52 MGTC in the basement garage. That little car took over a year to fix with three of us working 3-4 nights a week for 2-3 hours. It sat for 2 months after the last coat of primer was sanded. It sat for another month after it was sealed. Eventually, we put 50 or so coats of DuPont black lacquer on that car, carefully hand sanded with 600 grit between coats. A solid two gallons of paint was sprayed on that little car. Hell it sat for 2-3 months after the last coat, so it would dry and shrink. Then it was sanded starting with 1000 grit, and all the way up to 1500 grit. Then it was hand rubbed. Dad's final secret was cornstarch, he'd sprinkle it in with compound. After another 60-90 days, we waxed it with pure yellow carnuba.
I never could make a living at it. I did , out of necessity, do it as part of my work at a shop where we did custom vans for the handicap. That's where I learned a few short cuts.
