None of the strength is in the sheath. The sheath is just there to give your rope grab or hitch something to bite on. The sheath tends to unravel when it gets a nick. If you get in early, you can seal the nick with any bit of steel heated up red hot with a flame torch or even your gas cooker. Use a hobby knife or whatever. Sar the end, and squash it flat while its still warm so it will pass through your rope grab smoothly.
After 3 years though, you probably ought to just buy a new flipline. What do they cost over there, $50? Even here you can pick them up for about $75. I replace mine pretty much every year. You are guaranteed to nick the new one on the first day you use it.
Shaun