I use a lot of copper wire from old wires, bulk spools, etc. I did notice that some of the wire sold as fly tying wire does not tarnish as quickly.

When I use wire from old cords, electrical motors, etc for patterns like brassies and copper johns and similar patterns I coat it with Sally Hansens Hard as Nails or other lacquer. No more oxidation- or at least it is held at bay for a long long time.