a) Use a thread paste sealant or (b) a teflon tape sealant.
The right sealant for threaded joints is non-hardening,
compatible with plastic and doesn't add slipperiness to
Do not over tighten. The recommended way to assemble a Threaded
Pipe Fitting is finger tight. Tighten until firm with spanner
or pipe grip. One to two turns should be all that is required.