Sealants for parallel threads: PTFE tapes in several thicknesses and roll lengths, plus liquid thread sealants with different curing speeds, some with DVGW certification and BAM approval for oxygen.
When needed: Parallel threads without their own sealing element always need a sealant — this applies to the reducers and double nipples as well as to the VSG plugs. Tapered threads and versions with O-ring or copper washer, by contrast, seal by themselves. Liquid sealants cure in the thread gap; depending on the product, the connection is tight immediately or can only be loaded after curing.