Gotta love the clear coat.

Always use a sealer! Your order should be...
(Always read your product data sheet)

Etching Primer over bare metal to seal the surface
Urethane Primer to cover Etching Primer and used to level the surface with sanding. If you sand through to bare metal, guess what? Re-prime.
Sealer applied about 30-45 minutes before base coat (pick your shade of sealer because most new paints don't cover worth a crap now.)
Base Coat or Single Stage
Clear Coat over base within time per the data sheet.

Old single stage paints and primers were usually lacquers, old enamels couldn't cut into those lacquer paints. You can reduce an enamel with lacquer thinners, but you can’t reduce any lacquer paints with a enamel reducer.

Most newer paints are urethane or basically catalyzed epoxy and are made to adhere to a sealer.

Paint is part solvents. And The solvents will basically soak into the old paint or primers, depending on what surface you are spraying over. Once the solvents start soaking in, they will lift the old paint or primer almost like a paint remover. Sometimes it can happen instantly or take up to a few months or years. But you will see it and this can start peeling and flaking...