I'm trying to plumb up an old cast iron radiator. I've got 2 new 1
1/2" threaded inserts which screw into the body of the rad. Into these
go the valves.

I've used PTFE tape on the threads but don't seem to be getting a
sealed joint on one end. Is there a technique for this? Alternatively
there's a cement called plumber's mait, which I think might be the
sealant originally used on cast iron rads.

Is this any good?