Harald wrote:

We're filling a Penn reel with dacron backing, and plan to top up with
a couple hundred yards of fireline. Which knot will do best in
preventing both slipping, and one line cutting the other?

TIA,

I use the double nail knot to join all "spectra" lines (fireline is
Spectra" to mono, it has never failed me, I even use this for shark
fishing. This knot will even work better with SPectra to Dacron

It is also the smallest diameter knot that two lines can be joined with

Here is directions using my tool (no longer available, but you can use
other tools like the Fast Tie)

http://ezknot.com/line2line.htm

--
Rodney Long,
Inventor of the Boomerang Fishing Pro. , Long Shot rig
Stand Out Hooks ,Stand Out Lures,
Mojo's Rock Hopper & Rig Saver weights,
and the EZKnot http://www.ezknot.com