In 50+ years of modeling that included a very close attention to paints and painting and especially about fuel proofing, the only thing I've ever read or heard about fuel proofing will probably make you laugh. The info on the fuel proofing agents has not been obtainable as far as I know. Except for the one thing.......

You ready for this?

You used to be able to make the dope you could buy at airports fuel proof by the addition of banana oil. Yup. Back when fuel proof model paint wasn't readily available, you could make your own. And the fuel proofer was banana oil.

I don't think that is toxic.

And back in the 70s and 80s, before the more exotic paints came along, we would still use banana oil as a retarder (for blushless coats in humid conditions).