Thread sealant (aka “pipe dope”) is formulated to seal threads. But it’s great for almost any connection, even if the threads don’t form the seal. Use it on compression fittings, ground fittings and rubber seals. Because it’s slippery, it allows connections to slide together correctly for a good seal. And, if you use a type that doesn’t harden, disassembly and repair will be easier years later. Some types of dope harm plastic parts, so check the label.