"Loon Knot Sense & a drop of Zap-A-Gap to the very end of the fly line which had been sliced on an angle to prevent water intrusion."
I always thought that water migrated into the end of a fly line until I met Leon Chandler, who spent 51 at Cortland Line Company and retired as Vice President. I met him on the Missouri River at Craig. He was spending an entire summer there in his camper, fishing from a personal watercraft.
I'm pretty sure some of the photos at the very end of the article below were taken on the tailwaters of the Missouri River between the Holter Dam and Craig, Montana.
Leon passed in 2004.
I asked him about the water migrating into a fly line and he told me that Cortland did experiments, submerging fly line in water and found that there was very minimal water at the very end of the fly line core. It cannot sink the fly line.
I gave this some thought and concluded that for water to enter the core of a fly line it has to displace the air in the spaces between material comprising the woven core. Where does that air go? It cannot escape since any water at the tip end of the core would act as a plug keeping the air from leaking out. The air must stay in the core so any a small amount of water enters, the remaining air in the core keeps additional water from entering.
Can't the air migrate to the other end of the fly line attached to the backing and leak out? I suspect the knot tying the backing to the fly line compressed the fly line including the core prevents his. Plus for air to migrate, the water must be under some pressure to push the air through 90 feet of fly line, which it is not.
Secondly, for something entering the core of a fly line to cause it to sink, it must have a specific gravity greater than water. It must be heavier than water. Since water cannot be heavier than water, water cannot make floating fly line sink in water! Since the water does displaces a bit of air, it can make the fly line (tip only) FLOAT A BIT LOWER but CANNOT SINK the line.
Water has a volume and CANNOT enter a CLOSED volume unless it displaces an equal volume from the closed volume OR the water is under pressure so it pressurizes the volume. Since water at the surface is NOT under pressure, it cannot pressurize the volume of any air in the core of a fly line.
So water CANNOT migrate into any CLOSED container. Since the interior of a fly line is a closed container, significant water cannot migrate into a fly line from an exposed tip.
There is no harm in sealing the end of a floating fly line, but it really is not necessary.