Hot applied coal tar enamel with 15# felt bitumen
Absolutely Not!
natural gas line
No.
It is Natural Gas Pipe Line Regulator
It is Natural Gas Pipe Line Regulator
Yellow
Yellow
Use only black pipe
I assume you mean black iron pipe, which is used to run the gas line to your furnace.
Either shut the gas off at the meter or install a valve on the pipe. As long as you don't have an open flame somewhere close to the gas line and have everything ready, you can take the current flex line off and install the valve with little gas excaping. Use pipe dope on the threads of the pipe instead of Teflon tape. This lets you cap the pipe with your thumb while you apply the pipe dope. Then just install the valve and tighten, making sure to hold the pipe so that you don't disturb the next joint further down the line.
Fill pipe?
The gas in the line contains a proportion of water vapour (which is also a gas). When the gasses in the line cool below the dew point for the gas water vapour mixture, water condenses out of the gasses inside the gas pipe.