I only used primer when the color wouldn't cover well. For example the maroon on hiawathas came out a candy color on bare metal. The gray and orange covered well. I put a thin coat on, then a day or two later, did a thick coat.
Just a thick coat results in fish eyes. The first thin coat provides a uniform bonding coat, and if a fish eye develops in the first coat, I could scuff it and fill it in with a brush before the second thick coat.
I started repainting trains in 1977. Mostly just stuff to play with, a few peices a year.
Then I repainted a lot of trains in the period 1989-2002. This was done for money. Back then you could make money doing it. I'd buy a 97 coal loader that was badly scratched for $10 and repaint it with the paint I had, and get a hundred for it. Or a City iof Portland set for $75-100 and sell it for $300 after painting it. I also did a lot of frames and steam chests on steam locos.
I used custom matched automotive enamal that the store packed into spray cans for me.
I stopped doing restorations when my business took off, and the prices of repaints went down. In about 2002 the prices for good repaintable trains went up because of ebay, but the selling prices of repainted trains dropped.