Blue Max Film Splicing tape
I use it on my bulk loads, and also to join two 120's together end to end to load onto a single paterson style plastic reel. It is not all that sticky on anything other than film base, and it really sticks to itself. It does not loosen while in contact with processing chemicals.
On my 35mm bulk loads, I cut the film off the spool, rather tha try to get this tape off the end of the film in the dark.
My roll was found at Burlington Camera for $20. For that price f one 1" wide roll I have what I would estimate to be three lifetimes supply of the stuff, at the rate that I use it.