The non-setting jointing compounds (e.g. Hawk White) are for use on metal compression fittings and tapered thread fittings. Not seen them in TH but not really looked.
Other options are PTFE tape and liquid PTFE. There are some good silicone sealants too. In UK I always use PTFE tape on tapered threads and Fernox LS-X (silicone based) on everything else that needs sealing - although most times if you make the joint correctly you don't need anything. In Thailand only used PTFE tape on the plastic treads.
The epoxy putty linked above would make a permanent joint.
