Nail knot is pretty foolproof and honoured by historical long use... Though it is dependent upon / vulnerable to the bonding of outer to inner core. I've tied a couple on which, when tested, cheerfully cut through the outer surface and stripped it, sliding off the end of the inner core and goodbye fly line and fish if that were to happen in real time use. So not a fan really.
A figure eight knot into a loop tied on the end of the backing works well, the resultant lump doesn't matter much with fly lines. Though it does give a lumpy / bumpy moment or two going out through the rings if they are too small.
Particularlymake sure the knot isn't too big to pass through your tip ring. That kind of mistake may not matter with an eight ounce brownie but if you have hooked thesort of fish that can pull out an entire flyline and still keep going, ie.a Salmon, Sailfish, Bonefish or Tarpon, then it can mean saying goodbye to therod as wellas the fish !
Or you can use the same sort of braided loop you use at the sharp end.I took a long time to come round to it as I feared the braid 'relaxing' and contracting in its long rest on the reel in between duty sessions, as hardly ever put under tension(unlikethe sharp end)- how often do you get down to the backing ? honestly..... ? - and then, when called upon to do its duty, yawning a bit,turn back under the duvetand slipping off.
But after a little thought I nowtake a few cunning steps to ensure that my braided loops don't ever slip and I do use them on the back end too;
i) make sure its all the way on,
ii) pull as tight as you can so the old finger trap type effect is working at max strength and then put three or four dots of super glue along it an inch or so apart - this has the effect of gluing it but without making it stiffand unsupple -
then, (and this is the clever bit) iii) instead of using one of the manufacturer's standard slide over sleeves to lock it I use a half inch piece of the thinnest tightest piece of shrink wrap tubing I can get to slide down over the braid, not quite to the end of the braid, then dunk in boiling water to shrink as tight as she will go,
then (double clever bit) iv) slide down a three inch piece of slightly larger diameter shrink wrap tubing that covers over the braid, the first shrink sleeve and the loose ends of the braid on to the line.... then ditto shrink over all, aiming to have the first thinner shrink tube just about dead in the middle of the three inch piece.
Thin, flat, neat, rod ring friendlyand never had oneof these come loose or off so far. And if you want to be really clever at the sharp end you can get shrink wrap tubing of different / contrasting colours which doesn't half help bite detection - black or flourescent orange on a yellow or pale floating line is good. Or white on a blue inter.... black on a transparent etc etc, your choice.
