[1930-Pickup]

I haven't seen much conversation about the hole itself.

This 90 year old 1/4-18 NPT hole most likely has rust, debris, and gasoline contamination. No matter what you use to seal the connection, the gas tank hole needs to be clean for the sealant to attach itself.

What are you doing to clean the hole?
In my experience, this process works well:
(Assuming the gas tank is empty, or close to it, and no longer leaking gas out the hole)
1. Use a non-ferrous bottle brush to scrub the threads clean.
2. Use a clean rag dipped in acetone and twist the rag into the hole. Rotate the rag, then pull it out.
3. Repeat step 2 until the rag comes out clean. This may take a few times.
4. After the acetone has evaporated, the hole is ready for the valve and sealant.

A related quick antidote:
My brother-in-law is a landscape architect. He says that people spend hundreds on a tree, but do little to prepare the hole in the ground. Then the tree dies.
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