That kind of paint separation can be due to a number of things. If the stripped cabinet was sanded with a high grit, the undercoat has nothing to bite into. Also, the first coat of undercoat should be thin, so that the next coat "binds" properly. The completed undercoat should sit (acrylic) for 24 hours and any reaction observed before sanding 800 Grit (highest), and then I like to pass the air gun over the top to see if there's any lift. Next, the base - which is mist then heavier. Let the cab bake in the sun. Then do the stencil colors.
It appears the base had nothing to bite into. But as you have found out - sometimes walking away and having a think how to fix the issue is the best course of action. You did a great job with the repairs.
Knocker - I might have THE VERY knocker that came out of Dad's Cosmic Princess - Donated to your project. The beehive housing / hardware we have new and the plunger itself - I'll need the measurement - we might have an original Stern hanging around. They were a dull aluminum finish.
