I know some won't like this but a file does a nice job. Polish the bottom with a fine stone, mirror finish, than shape the top bevel with a file providing the steal is not to hard, follow up with a stone, I don't. It is like an axe, it has a long handle and therefore leverage and force, whereas your chisel is for fine tuning and sharpened finely. My adze leaves a polished surface which is all I am after and only takes a few seconds and back to work. Tim