i have always had that kind of trouble out of the duplicolor metalics so i don't know what to tell you, last time i tried them i did wait over 24 hours before the final coat .. and it still looked milky ... all i can figure out is it just has to be done in numerous small light coats with no thick coat ...
i even made sure the can was shook up and heated and such just in case that was the issue (have had several duplicolor paints .. 'seperate' on me when they cure... i have since stopped using em)