I happened to be rereading the article I sited, looking for something different, when I happened across the following statement: "Water resistance of a piece of wood is not enhanced by deep penetration. Wrapping wood in plastic makes a pretty good waterproof seal without any penetration at all. Likewise, an epoxy coating on the surface is more water-resistant than a thinned epoxy coating that has penetrated deeply into the wood because, in most instances, the epoxy thinned with solvent is porous." The people at West Systems have concluded that thinned epoxy is porous. That seems to be a pretty compelling argument against thinning epoxy being used for fuel proofing.