The system may need draining beforehand if the leaking radiator is on
the ground floor. Remove the radiator gland nut with a ring spanner
and remove any tape or packing. Using graphite yarn, tie a knot around
the stem and coil it tightly clockwise. Some force is required on the
gland nut to compress the yard before the thread will engage. Once
tightened, turn on the heating after checking the pressure gauge and
bleed any air from the radiator. I have found graphite yarn to be far
more effective and durable than PTFE tape. Silicone sealants such as
Fernox LS-X are fine for sealing the lockshield valve but no use for
the wheelhead valve.