Seal it with a Black neoprene washer!
Neoprene Washers are used with a Hex Nut or Wing Nut and Tail Piece when attaching to a shank or coupling for a leak-proof seal.

California Proposition 65

⚠ Proposition 65 Warning: This product contains chemicals known to the State of California to cause cancer and/or birth defects or other reproductive harm.