They fix it. [Dotanuki motions to his sword, and then to one of the many scars on his body and face.] And sometimes it leaves a mark and sometimes it doesn't.

If they buff out an old scratch, some of my scars disappear. It's a little weird when that happens.

[Aka, not like a doctor at all, but he can't really talk about how it works. It's a little weird to most humanoids.]

Otherwise, you repair me like you'd repair any sword: buff, polish, and sharpen.