Mom had a coworker that took a nice bike and randomly spray painted it with multi colors to make it less desirable for thieves, while still a good commuter.
Apparently you can get rusty pipe decals. From another thread:
Originally Posted by
smasha
I'm actually considering re-painting a brand new commuter bike to look like rusty pipes, to discourage theft. Maybe something like semi-gloss grey, with splotchy matte black/orange/red lines and blobs.
Picture here is vinyl stickers made to look like rust.
Of course that might not work if it isn't actually steel. But, a decal may also be reversible should you wish to take it off. I don't know how you could ensure good coverage.