I was watching Trucks TV on Spike and they were talking about this same thing. How to make Copper plating look older. They recommended using Miracle Grow African Violet in liquid form and this will give you the aged look your looking for.
Here is a link, it's at the end of the show.
http://www.spike.com/full-episodes/1...eason-2-ep-234
Truck looks great, keep up the good work.
Nick