Rather than use the serrations I'd suggest using the edge of the spine. Keep in mind the intense localized heat created by the firesteel will actually create deposits on the blade (I suspect these are actually small pieces of the blade re-welding themselves to the knife).

I recently used my Gerber LST folder (uncoated satin finish blade) to strike a firesteel and was absolutely convinced that I had ruined the knife -- the entire side of the blade appeared burned and sooty, and was very rough to the touch. Thankfully some scrubbing with an SOS pad cleaned it up.

Jim