Leaking Petcock '86 VF500: Is a gasket missing? What gasket maker to use?

What kind of gasket is there supposed to be between the petcock flange and fuel tank? I started to refill all fulids after finishing assembly of my '86 vf500 rebuild and gasoline started dripping out between the petcock flange and the fuel tank. The only gasket maker I could find that said it was gasoline resistant was Permatex Motoseal. I used it on the interface last night, and shook the tank to slosh the fuel against the petcock and the there is no evidence of a leak. I'm surprised that the motoseal is not stiffer or harder after fullly cured. Anyone else have experience with motoseal in contact with gasoline? I think I am going to buy a couple of baby food jars to run an experiment with various gasket makers sitting in fuel overnight to see how they hold up.