I have actually bought two cans of this stuff:
Weicon Copper Spray
It's not exactly cheap, but it's very fine real copper particles suspended in an resin cement
matrix (after application), which will behave pretty much like pure copper with regards to
tarnishing and aging. It's basically a way to cold plate your parts with real copper. I'll be
using it for various small parts on HELIOS for exactly that purpose. I looked around for
various alternatives but I didn't like the look of "fake" copper sprays (i.e. sprays that just
look like copper but are really something else).
However, I haven't yet tried it so I can't yet say how well it really performs or how good it
looks. They also make steel particle sprays and other sorts metal sprays, it's pretty high
quality stuff. There's also another company that makes a similar product, Wekem IIRC.
EDIT: I could only find this on eBay, no store in my country had it (at least none I
could find). The seller I bought it from ships to the EU and Switzerland.
link