The welded loop on my rio mainstream line has started to become undone at the connection. I think it happens because the leader hits the line and slices between the very tip and the line it's connected to. I was going to brush some superglue on the connection, but want to make sure it won't make the line sink or the coating wear away.

just cut it off and use a nail knot or go buy one of those pre made loops that look like the old chinese finger lock things with the heat shrink tubing over the connection.  I have used both methods and they work well.  Knowing how to tie a nail knot though can save a trip if you are in the middle of nowhere and what you describe happens.

 

I don't think superglue would hurt anything though but you might want to test it out on an old piece or something first just to be sure.  I vaguely remember the owner of the fly shop i used to work at putting super glue on the chinese finger lock type connectors but am not 100% certain.

 

Also if you use a nail knot you can just use a heavy piece of mono and tie a perfection loop in the end and use a loop to loop connection to your leader.