Here's the finished tank. Third time's the charm. It took a lot of restraint to keep from making it too perfect. For some reason, even without a flash, the copper shines through more in photos than it does in real life and accentuates the mottled effect. There will be a little more detailing yet.
To get the tank to this stage I painted it black then copper then dark brown and then sanded off most of the top layer with 220/400/fine steel wool. Then I painted along the "seams" to make it look like it was soldered - black, then gray. I also painted rings around the inlets to make it look like they were soldered on. I added a few copper highlights and a wash of turquoise in some areas to finish it up. The trick is going to be to make the rest of the bike look 100 years old, too.