My Bronco KV-85 came with copper wire to make the tow cables. Seems to be easy to work with. I primed it (mr surfacer), let it dry overnight, and then painted with Tamiya paint. The paint seems to be coming off easily down through the primer. Is this fixable, short of stripping it and priming with something else?

Try “Blacken It”. It’s a chemical blackener designed for hobby applications. But you only need to let it sit in the solution for a few minutes, when it reaches the desired patina, remove it and immediately rinse in clean water. You can leave it “as is” with a coat of clear flat, or you can paint it with your chosen paint, which will have no problem bonding to the surface. Make sure you thoroughly rinse, or the blacken it will continue to etch the wire. If you don’t overcoat with clear flat, you may see some green corrosion over time. I think there are other versions of this product from other manufacturers too. It can be found on line, in model RR shops, or at the various hobby stores like Michaels and Hobby Lobby.
VR, Russ

Agree that patina is probably your best bet. Something as flexible as copper wire I doubt you'll find any paint that will not crack and peel when bent. Perhaps an etching primer might help, but I think really if you want to paint it, you're going to have to first get it to the shape you want it before painting and avoid handling it as much as possible after painting.

My $0.02...

I use copper picture-hanging wire for cables, and find it best to bend & fix the wire to the vehicle before spraying the whole thing with spray primer - then the paint stays on because the wire is not being flexed. Beware using washes though, as they tend to run down between the wire strands like water through a storm grate!

I use Mr. Metal Primer.
I prime the metal(PE, wire) part, wait for couple of hours, up to a day, then spray the Mr.Metal-primed part with my primer of choice.

The other way around with copper wire towing ropes-I first cover them with solder and then use Mr.Metal Primer.

HTH,

Forgot to mention my go-to primer is grey autobody primer spray in a rattle can, from the local autoparts store, so it grips metal much better than anything in the hobby meant for an airbrush. You could use an etching primer, but beware of getting it on the plastic...

Look up Rustall™. "Rustall will simulate rust on styrene, metal, cast resin and even paper."

Even though copper does not "rust," the base coating of Rustall wouldn't care.