There is a small natural gas leak that persists at the black iron union joint on the short line between a local gas shut off valve and the furnace. The leak is so small and the union joint so hard to bubble test that soap bubbles just don't show the leak clearly, but the odor is definitely there.

I already turned off the gas, disassembled, cleaned, and sealed all the threaded straight iron pipe parts with liquid pipe thread sealer. Lastly, I reassembled the union joint, carefully aligning everything first. But the slight odor remains right at the joint.

I am pretty handy. Do you suggest I replace the union joint? Then try again?

In my area of the world the local gas utility uses a form of nonhardening putty which they push into the threads on an already made joint to seal a very small leak. When I first seen them do this I did a dubble take since I felt that was a bit unsafe.

Replacing the union is a option since you seem handy with tools. When you go shoping for a union try to find one that is of good quality with a carefully ground face and seat area so the problem will not happen again.