Boiling a browned frizzen at 212 degrees (approx.) isn't hot enough to affect the temper of the frizzen. By not applying solution to the face of the frizzen you won't have any worry there either - not that that would affect it's abillity to spark. The flint would eventually just scrape its way through the layer of rust to clean metal or you could touch it on a belt sander to remove it in a juffy.
