I've used nail polish to patch up a paint job on one of my bikes. It's held up nicely and is hard to see (and I used a LOT of polish!!). Actually, the only problem I've had with it is that the polish seems to collect dust faster. I intend on repainting it... but perhaps a coat of turtle wax would stop that dust thing.

..I used it mostly because I've a ton of it in the closet and it's what I'd always used on my car.