You don’t need much of anything. The thread pitch of the BSPP fitting is slightly different than the euro metric thru-hull so it self seals. And it threads WAY down on the fittings. For regular duty teflon tape works well and facilitates disassembly if and when you need it. For heavy duty a standard fiber added plumbers pipe dope work great. I might use that on an engine fitting where vibration is present.