Scrape or sand the black stuff off and the rods themselves all work pretty easily when needed.
Last year I tested a bunch of brand name ferro rods against their cheap and nasty Chinese clones in both "workbench" and field conditions and found very little difference in the efficiency of the ferro rod itself when used to light dry stringybark bullswool, although the quality of the things it was glued to like plastic handles and "magnesium" bars and such varied wildly.
The only caution I have to add with using a ferro rod to light a metho stove is to make sure that the person using said ferro rod is adept in its use lest they knock over their Trangia burner trying to "get the b@$tard to light!!!"
