I have used ZR exclusively on my 54 from frame to body but on the previous advice of Bruce I did not use it on the top surfaces of the body with the exception of one panel.
The rear panel I transplanted to the 5 window cab had been previously stripped to bare, metal prepped and then coated with black ZR. It sat this way for about two years before I removed from the old cab for the transplant. Once the panel was on I scuffed it with 220 grit and then primed over that with Epoxy Primer. It was another year before I did further prep on that panel for the top coat which included additional sandable primer and then a final seal coat of epoxy primer before the color was laid down. I do not believe I will have any problems with the paint system but I would not use ZR as an initial base without letting it cure very well over a matter of months then prepping it as I described.
ZR Rocks in my opinion.
