I just used that teflon thread tape plumbers use.( Wind it on the brass thread anti-clockwise)
It ain't entirely fuel resistant, but had no problems and I think it's mainly due to the tapered thread on the brass making it seal.
I also smeared the thread tape with Stag Jointing Paste or use similar hard setting gasket paste after I wound it on the brass thread.
Just make sure it's not prone to fuel.
Cos the tank is questionable HT quality, I play it safe on THEIR threads. Don't over tighten, or you might end up cracking the tank's thread part cos of the taper.
Let it set and dry completely then add fuel to the tank.