Some frizzens are made with lower carbon steel that is simply case hardened to get a good sparking surface. When this thin hardened skin wears through, the spark quality is diminished, and rehardening may help.
Refacing is done exactly as Mike describes. In one gunsmith book I read, the author recommended using a small thin old file as a source of material for the sole. He would grind it to shape very slowly with frequent quenching so as not to overheat it and draw the temper..
Metallurgists often use a simple test called spark testing to judge carbon content of a steel sample. This is done by holding the steel against a grinding wheel, and observing the color and quality of the sparks. A through hardened file made of good high carbon 1095 steel will provide a good shower of white hot branching sparks, which is what you want falling into your flash pan. A file made of a high speed tool steel will give weaker and more reddish color sparks. Many people think the spark comes from the flint. Actually, the sharp edge of the flint shaves off particles of steel and the speed and friction of the cock falling makes them white hot.
