....paint it on!
I guess the other way of putting it could be oh ****! I've ruined my cup... what now?
All of the browsers (and lurkers) of this board (speaking of which, where are you Sean?) will be familiar with my good friend Bruce (BCS), as well as being one of the nicest and most genuine guys you could meet Bruce also very generously contributes articles and reviews to my website from time to time.
Bruce has written a great article on re-finishing an alloy chop cup that started life as a faux copper finish.
You can read more about it here;
http://tinyurl.com/37pzf8g Bruce has actually painted a patina on the cup and though the pictures don't show it perfectly, perhaps the best view of the photos on the site is on the bottom of the cup in the 7th photo.
Bruce, thank you for your friendship and for contributing (again!) to the site.
Bri