as others have mentioned, the material of your dropouts will determine whether they're gonna rust or not. If you have steel dropouts, but they're *noT* stainless, they will rust. some grease will help slow that down. If it looks pretty bad, you can buy some nail polish in either a matching or complimentary color (or, hey, you can use an ugly color, too) and touch it up. Just make sure you let it dry thoroughly before reinstalling the wheel. And, now that you're aware of what can happen while installing a wheel, try to be a little more tender with it. You're always gonna eff up the paint in the dropouts when changing wheels, but a little bit of care can minimize the damage.