Another newbie here learning to use the zinc kit...
I've been using brightener and still get dull looking parts. My power supply is far from optimized so that's likely part of the problem. The brightener makes the zinc metal smoother but the finish comes out with a brown surface finish. At any rate, what I found that helps is a brief acid dip (phosphoric acid) after the parts are plated to remove the brown residue. There is still a slight brown tinge but the dip really cleans up the streaks and such - I only dip them for about 10 seconds or so. You might want to try something similar on your cad copy parts and see if this improves your results.
