conductive paint + water =?
I'm not sure if conductive paint will tarnish - but I do know that the Graffiti Research Lab uses conductive spray paint to add tracks for LEDs around their work.

From what I've seen some of the work is on a regular exterior building walls.
Even if there's no information on the website about your questions, I'd imagine they would probably know - maybe you should contact them if you don't have any luck by other means