How do I sharpen a Japanese pull saw?

I have a Japanese-style pull saw that's starting to dull. How do I sharpen it?
—John Eisenberg, Toledo, Ohio

It depends on whether the saw teeth were hardened during production, John. Some Japanese saws have impulse-hardened teeth, where a high-frequency heating technique hardens the teeth but not the rest of the blade. These teeth stay sharp several times longer than non-hardened saws; but because they are harder than sharpening files, they can't be resharpened. How do you spot impulse hardening on a saw? The teeth tend to have a bluish discoloration different from the blade body, and if you draw a file over the teeth, they'll scratch the file.

If your saw was not factory hardened, you can sharpen it using a specialty tool called a feather file. Feather files come in several sizes for different tooth counts. The two most common are a no. 3 (75mm) for dozuki saws (reinforced spine, fine teeth for crosscutting), and a no. 4 (100mm) for ryoba saws (dual-edge blade for crosscutting and ripping).

To sharpen with a feather file, clamp your saw in a vise with the teeth just above the jaws. Because these teeth are sharpened in an alternating left-right pattern, you'll sharpen every other tooth from one side, then flip the saw around and sharpen the remaining teeth. Push the file across each individual tooth 3 or 4 times, more if necessary to repair damaged teeth. After completing this step, file tiny secondary bevels on the tips to avoid having them break off during use.

Source:
Japanese saw files (set of 2): part no. 06Q10, Woodcraft Supply, 800-225-1153, woodcraft.com.