The petcock attaches to the tank with a compresion type fitting so using thread tape wont do anything, other than maybe reduce friction allowing you to get it on tighter. Even if the threads are sealed, gas would still leak under the collar.
Have you put it together without any oring? It should seal the gas "OK". It will still leak but not alot, not pissing fast, probably not even drizzel but drip drip would be expected.
If it really comes through without the oring I would look closely it see if the male tank fitting is damaged. Alot if times people take the tank off and set it on the ground and damage the mating surface. When taking the tank off and setting it down, it should always be supported by something OTHER than that spot. Just put 2x4s under the tank to support it. If you dont do that, you get leaks....
Also, your petcock should ALWAYS be off overnight. Even going into get a loaf of bread. Its a really sad, sad sound to hear your engine hydro-lock because a cylinder is full of gas. Then when you do get it unlocked, guess where all that gas is? Yep, in the crank case. BTDTGTT.
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