From a friend of mine who is a grafter AND a knife-maker:

Sharpening: A hardware store variety of carborundum stone can be used, however it is really only useful for coarse reshaping, even the 'fine' side is quite coarse. If you have a knife that is in good order, try this to maintain it.
Acquire a flat surface, at least 2" by 6". I used a 6" x 12" square of high grade plywood. One could use thick glass, plexiglass, masonite, steel, whatever. Buy a can of all purpose spray adhesive , spray the back side of wet/dry sandpaper that is cut to fit the flat surface (don't spray the board or it may be difficult to remove and replace the sandpaper). On my 6x12 plywood piece I glued down 4 grits 220 400 600 1200
With this homeade 'whetstone' you can now lap your knive's flat side truly flat. Be sure to have a good incandescent light nearby so you can examine very carefully the bottom side of the knife, there will be a microscopically thin reflection of light right at the edge, keep honing till it is gone. To hone the bottom side perfectly flat, push straight down on it with a finger while moving it around on the sandpaper. I see no reason to use water or oil on this sandpaper strop. 600 is the grit I use the most, 1200 is to give a slight microbevel to the top side, and as a final polish to the bottom side. I rarely use the 220, as I have diamond lapping stones which cut more efficiently