DMT's Dia Sharp sharpening stone is only the size of a credit card yet will put an edge on any knife blade up to 4 inches long, plus hatchets, fishhooks, and anything else that's dull. It comes in extra-fine, fine, and coarse grits (fine is the one you want), and there are 4-inch-long and 6-inch-long versions as well. Just wipe it clean--no oil needed. About $10.50 [BRACKET "phone"] 800-666-4368; www.dmtsharp.com.
--David E. Petzal