These are cheap, sharp, easy to maintain, and you don't feel too bad if they get lost. I lent my son my suitcase, with my previous no.8 in the inside pocket by accident and he made it through three airports before the TSA in Chicago confiscated it. Apart from the memories it held, it was no huge loss and i had it replaced the next day.
flip it upside down so that you can see the unglazed ring around the base. just swipe flat across that ring like it was a whetstone.