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.