It's only a failure if you don't learn from it. Don't ignore lessons coming at you hard and fast. I understand that the process may be more than half the fun for you, but don't ignore what's happening right in front of your eyes.
I've
never seen a successful color change on the E-body door-panel type of plastic (most kick panels, '71-'74 B-body lower door/sail panels, etc). Black panels hold paint a
bit better (they don't turn into chalk like white and colored ones) but they still don't hold paint well. You might well be the first, but forgive me for doubting so.
My Challenger came with
four sets of door panels: Three sets were painted (white to black, green to white, and green to black), and the "native black" panels were both rough and cut for speakers. All eight left in a garbage truck. I was wasting neither a minute nor a penny on what I knew would be a temporary fix to a permanent problem. In my experience a watch, wedding-ring, sleeve button, or even car-show wrist band will peel paint right off that stuff. Unless you're a "gently push until click" door closer, the average shut will start the finish to separating in fairly short order. You're already seeing this.
@restoman suggested using adhesion promoter and flex agent, which might've prolonged the failure process a bit. He's much better versed than I, but I remain unconvinced there's a viable solution simply based on the nasty nature of that particular plastic.
I admire your determination, but how much money are you going to spend attempting to not spend money? How much time are you going to waste on this that could be used successfully elsewhere? There's still a lot left to accomplish, including spending some quality wheel time in your hot rod.
Either find a good set of used panels (they do exist if you look for 'em) or pop for the repros. Move on.
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If (
when?) you decide to suck it up and buy the reproductions,
make sure you get the injection-molded ones,
not the vacuum-formed panels. The originals were injection moldings; vacuum-forming is a less-expensive way to make them. I've held both in my hand; injection molds are much better (my car came with a set of
new vacuum kick panels--they left in the same truck as the door panels). The vacuum-formed parts look like a high-school sophomore's failed art project. Yeah, it's that dramatic. The injected parts have a sharp grain similar to original. Vacuum parts have a texture like slightly lumpy Cream of Wheat, but with less detail.
Vacuum sucks, pun intended. It'll help you remember should order time arrive.
Strangely enough, looking at Year One just now, the injection-molded ones are considerably
less expensive. In fact, the prices on the better panels haven't changed since I bought mine ('15 or '16). At that time, the suck-shaped parts were
hundreds less than injection molds--and
still not worth it.