Splicing lines has been wonderfully covered on the old INternational Spey casting pages, which are now archived here on FFF.
Go to FFF search engine.
type splicing spey lines
scroll down to a thread called "splicing spey lines"
I've found that aquaseal thinned with lacquer thinner works well for splicing. Zap a gap seals up too quick for me, and sometimes results in a wrinkled splice.