The only threads you need to put PTFE tape on are the rad tails out of the old rad or on the new ones if your replacing them. Make sure you wrap the tape on clockwise
"Need", no. But I wouldn't replace rads without some jointing goo of some sort on the unions. It costs nothing over a load of fittings, and it can make a difference. I used to maintain that it shouldn't be necessary, but experience taught me otherwise.
There's Fernox LS-X, a silicone compound, or any number of pastes.
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