Hey Tatsu...
What kind of sheath is it, Whats it for ??
I don't suggest anyone not knowing Exactly what they are doing heating up a sheath, especially in a case like this..Straightening waves is no easy task....
If not done properly,,you'd be opening a can of worms.
If its really bad ,it may be easier, just to get a new sheath,or just live with it.
Drop me a note personally if you wish..
ttyle
Eric...