I'm replacing a radiator and ahve removed the rad tails to transfer onto the new one ,i know its PTFE tape on the part that screws into the radiator ,but whats the best way to seal where the rad tail union joins the outlet/inlet (on and off) valves .
Everyone has their own way Ray.............if it's a 15mm olive type (compression fitting).....I usually use PTFE tape........if it's a conical seat then maybe a little jointing compound on the seat.......... :o)
Its the valve that comes out of the floor and bolts onto the rad tail that screws into the radiator ,when i've done this in the past i've used a smear of jointing paste ,but wasnt sure if this was ok or correct.
as said its a compression fitting so you shouldn't need to use anything on it at all but if you wanted to you could try some Fernox Lsx on the olive and the thread
I always use PTFE on any threaded connection in plumbing work - whether it's always necessary or not I don't know, but it won't do any harm, and you get that feeling of satisfaction when you manage to run a piece round 3 or 4 times without it getting all tangled up :-)