I had a measure of success using a product called "Seals all" which is resistant to petrol/gasoline. If your fuel as an apprecable amount of ethanol in it, results may vary.
If, by chance, you are able to find fuel without ethanol, you can rub bar soap on the threads, this bar soap conbines with gasoline to form a gel that seals quite nicely. This bar soap method does not work if ethanol is present.