"Why was it hotter?: I'm pretty sure because the heat into the bottom of each section was the same. Same air temps, same air velocity, same color (unpainted). While the heat loss off the top was much more for the painted sections, cooling them more than the unpainted section."
Very interesting, and this explains something I have observed first hand in the past, but never really given a whole lot of thought. Way back in my younger years, I had the miserable job of working at a tractor/trailer junk yard cutting up old semi's for scrap, in Phoenix, AZ. The summers were hell, and one of the first things I learned (should have been to wear a hat, young and dumb) was to never leaver your tools in the sun. They got darn hot, really fast, and the chrome tools stayed hot for a very long time. I only had to burn my hand a few times before I started paying attention to the movement of the sun to keep them in the shade as the hours went by.
I'll for sure be doing some back to back tests...