I used Ace Hardware Pipe Thread Compound TFE paste on the threads then tightened the brass fitting down.

Here's what the bottle says:

"SLOW SETTING: Brushable paste dries slowly non-hardening, withstands up to 3000 PSI (gases) at temperatures from -50 degrees to +400F and 10,000 PSI (liquids) from -50 degrees to +500F. Use on water, steam, natural &LP gas, oils, fuels and dilute acids. "

I'm going to spray soapy water on it in the morning to make sure it doesn't bubble up.