To form a foolproof end loop in a fly line, use the speed nail knot (see Tackle & Technique, July/August 2001). Fold the fly line over itself to form the desired size loop, take a 12-inch length of 8- to 12-pound-test monofilament and bind the two sections of fly line together by tying a seven- to eight-turn nail knot. Pull on both tag ends of the mono to firmly seat the knot. When the knot is really tight, the mono will dig slightly into the fly line's coating. Tie a second nail knot approximately 1/16 to 1/8 inch behind the first to make a loop that is as strong as the fly line itself. I've never had a loop pull free in 30 years of using this procedure. But for a superslick connection, tie a long, 20-turn nail knot that will completely encase the remaining tag end of the fly line. To provide some protection, you can coat the knots with a flexible adhesive like Pliobond or Loon Products' Knot Sense. I know many anglers who fasten a loop by whip-finishing the fly line together with fly-tying thread and a bobbin. This will work, but it's not nearly as strong as the nail knot connection.