I couldn't find anything from Duplicolor on this issue (at least regarding the Metalcast paints specifically), but 600 grit was probably too fine. I had been waiting for some decent weather here in the Northeast for months to get a decent basecoat on, but it has literally been snowing/raining/high winds in the area every weekend since last Fall. I got impatient and rushed the basecoats and had to stop and start many times. Also, when I finally got one nice day, I tried to rush the Metalcast coats and that was a disaster.
Chalk this up to purely user error. This is a great paint, but also an unforgiving paint. If you try to rush it or use it in non-ideal conditions, it will not go well.
I have Adhesive Promoter, including the Duplicolor brand. All the adhesion promoter I have on-hand says it is specifically designed to allow basecoats and primers to better stick to slick surfaces such as plastic and vinyl. I was not aware of an adhesion promoter that was designed to make color/candy coats stick to basecoats. I always operated under the assumption that was the whole point of a basecoat.