[Zanaspus]

I can't think of anything that won't also be harmful to the patina, but do this: Buy some beeswax from a craft store. Smoke a bowl of tobacco in the pipe, and while the pipe is still warm, rub the outside with the wax. Remove excess if there is any after the pipe cools.

This is something that should be done as a normal course of maintenance anyway. I suspect that after a couple of said treatments, you'll be happier with the look of the outside.