Just replaced six radiators here with new Kudox ones, been a job from hell. Got one radiator tail deciding to weep,did use pfte tape, was going to get a roll of LOCTITE 55 pipe sealing cord to try, what are plumbers using today instead of ptfe tape.
Loctite 55, I also put on a smear of jet blue (belt and braces I know). Not had one leak (ever). Get the Locite to go across the threads as well as 'in the threads'' . Works for me.
Loctite 55 for me as well. You need to 'damage' the threads by marking them with some pump pliers or similar and then run across and around the threads as suggested earlier. No leaks yet. Great stuff although it's messy if you have to dismantle as it shreds itself (I had made up a temporary stand pipe on a job once). It's also useful if you have to pack out a fitting such as a bib tap that won't quite sit vertically.