Never ever? Wow!
I use a "desperation method" of refreshing a flint in the jaws rapidly that is called "Hammering the Frizzen".
I'll warn you that this may be rough on the frizzen, but it's been done for centuries.
Set the gun on your lap or cradle it in your arms. With the gun unloaded, pull the cock to full cock and then manually lower it as the trigger is pulled (or, just start at the full-down position after shooting). Gently close the frizzen over the flint and then draw back the cock until just the last 1/16" or less of the flint is below the heel of the frizzen. Press down hard on the frizzen. The pressure on the flint will spall off pieces from the underside (this works MUCH better with bevel down flints, but will work either way).