Done back in Fall 2004 for my props when I cosplayed as Itachi for Halloween and MegaCon 2005. This was the first time I ever worked with resin and after much in the way of research and about $130 later, these are the results. I figured I'd share my experience, because it really was a fun and entertaining attempt.
Sadly, you can now find plastic/resin kunai at conventions for about $10. But this thing, it's SOLID. Not sharp, but SOLID...I'm sure with enough force you could stab someone, heh. But this is a good tutorial for any kind of item you want to make a resin copy of. I'd do it again.