Mono core fly lines are rare; I wouldn't worry about them. Paint stripper and nail polish softener will make the plastic coatings easier to remove. Don't let the line soak too long. A neat way to remove line coatings, including unsoftened ones, is by tying a short looped length of heavy mono (say 15-20 lb. test), double-wrap it around the fly line 1/2 - 1" from the end, the other end of the loop around a tool handle (to save your hands), and yank. Repeat until you've exposed as much line core as you need.