Hi Dave --
Sharpening of the flat carbides (as compared to the cup shaped carbide cutters) can be done on diamond laps, and as the laps are progressively finer, the edge becomes more fine .
There are some YouTube video by the DMT diamond lap folks that show how you can hone the flat and extend the life of the carbide.
There are folks that have the special gear to really restore the sharpness to new, but expect something around 80 percent or so with use of the diamond stones.
Warren Cutlery sells a diamond lap kit for finer grit polishing if you want higher percent of restoration.