I was attempting to add a short section of heavy mono with a loop to a WF5I clear mono-core fly line. My first nail knot looked good but, on knot seating/stress-testing, pulled the coating off the mono core and so failed.
I tied another nail knot after trimming back the damaged end of fly line and melting a little ball in the end with a flame. I don't trust it.
I'm thinking the best bet might be to whip a loop into the end of the fly line with some mono fly-tying thread after exposing a bit of the mono core. I'd use some Aquaseal in the process, too.
What's your preferred method?