The obvious answer is to use real diesel, but barring that you might try polyurethane. If I suddenly found myself in a situation where biodiesel was the only available fuel (and living in CA, that's unfortunately not beyond the realm of possibility) I would try some two-part polyurethane brushable mold material
EZ-Mix® 40 Brushable Urethane Rubber Compound In a molding application you'd use a mold release agent, but as a coating, urethane is a pretty effective adhering and should hold up pretty well as a barrier.