Robbie - some of the modern hi-tec/fine diameter coarse fishing lines are excellent for fly fishing - in fact that's where the development of modern monos has come from (via match fishing mainly). The only difference you may find with lighter, fine diameter strains of coarse match monos is that they tend to be deliberately quite limp/supple, which can lead to tangles if used as full leader material. Therefore I tend to use traditional standard mono (thicker) for main leader material (eg maxima ultragreen) and hi-tec fine diameter types for tippets/droppers, eg Preston Innovations Reflo Powerline - very strong for its diameter and also very tough. I use it for coarse fishing too. MAP Carptek is another. One of the first of this type was Daiwa Super Shinobi, which some fly fishers still use. Stroft is a similar hi tec mono sold more to the fly fishing market. Not only are these types very good, they are also very good value as they are usually sold on 100m spools, not the rip-off 30m jobs!
Your 12lb stuff is ok for thick leader butt section material but as Jim says, if you want to try furled leaders its too thick. I also think that Singapore style of furling is pretty basic and not very good. Seeing as "proper" ones are cheap as chips from Steven (mrtrout) anyway, I wouldn't faff around with the Singapore style.
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