I'm gonna open a whole can o' worms here....but why the hell do you want to use 1/0 or 3/0 thread on saltwater flies for????
I use Uni 8/0 and Danvilles Extra Fine Mono for 90% of all my Salt Water flies, and I tie up some pretty big flies at times on 6/0 - 8/0 hooks.
I've heard all the arguments about being able to secure materials better with 3/0 but IMHO and my own experience, I can gain just as much security within the dressing by using the 8/0.
I do occasionally use 6/0 when spinning deer hair, but the only times I will use 3/0 is for building up underbodies or heads of big flies or where a shaped, steep tapered head, such as that below is required, and even then, I finish off the process with 8/0.
I await the Flak
