diamond core bits are the way to go, with quite high rpms

and with water or water/detergent coolant


Quote Originally Posted by InMesh View Post
cast iron will surface rust in moisture prone areas quickly .
If you had placed it on the hot coals, open, the heat should have driven the water off as you intended, drying before it rusted.

With the lid on, the moisture was trapped inside, and had opportunity to cover every inch of the metal - and, heat speeds up the reaction, and you have a layer of rust.