FWIW, i ghetto painted a velobuild frame that i got at "primer" stage. It was primer with high spots sanded. I ended up re-priming so that the primer would be a solid coat, because i was getting bleed through otherwise. Paint is finicky. Cost me less than 30 eur for primer, 2 colours (the black is metallic, it's quite nice) and a cheap top coat (couldn't find 2K locally).
Given the effort involved, i'm happy with the result. Barely any sanding needed, because i got the frame at primer stage.

Before, after, and the bike i wanted to emulate.