Credit Card Sized diamond hone, 800 grit on one side, 1200 on the other.  
The 1200 grit is ideal for putting a fine sharp edge back on your carbide cutters.
Remove the cutter from the tool bar, place a few drops of water on the hone and invert the cutter so that the top surface is flat on the hone. Move the cutter in a circular motion until the edge is restored.