It seems like a compound question. That is, I don't know how to prevent the kurouchi finish from coming off (if that indeed is a problem -- I've seen one person say this on another forum, but not sure if this is a one-time thing). But the rust issue is only tangentially related. That is, maybe a kurouchi finish helps prevent rust, but plenty of carbon-steel knives don't have that finish at all, and preventing rust is a separate issue. And unless there's something particularly reactive about Moritaka's Aogami 2, it's not that difficult. You prevent rust by rinsing/drying when necessary, cleaning with baking soda at appropriate intervals, letting a patina form.
If you are asking how you prevent the finish from coming off just to protect the finish itself.. well, I don't have direct experience. I'm inexperienced with kurouchi knives in general (sorry!)
Edited by Wagstaff - 10/1/11 at 12:12am