Lefty: Most but not all mono core fly lines have acoating that under strain will strip off when connected to theleader with a nail knot. There are two answers. First, you can makea loop knot secured with two nail knots (use 10- or 12-pound-testmonofilament). When properly tied, this knot won't fail. However, Iprefer using the whipped loop knot because it is stronger than thefly line, and it slides through the guides more easily.Instructions for this knot can be found in Practical Fishing Knotspublished by Lyons Press.