I guess I didn't make my point clear enough. The oily water mixture is at its worst when the air is first released. Once the machine has been used for a while it will clear up or at least it won't come out of the hose nozzle as a "slug" of oily water. Once the mixture has come in contact with the aluminum, it is difficult to remove without the use of a solvent like acetone.

My experience in shipyards has been that cleanliness is a word that appears in the dictionary, but is used infrequently by the workers.

I had a project involving aluminum many years ago that involved chemical cleaning using sodium hydroxide followed by a nitric acid dip followed by a clear water rinse. The material was then supposed to be air dried. The workers took that to mean blow off the excess water and dry with compressed air. The compressed air was the kiss of death. Even with the oil and water separators used in their shop air system, too much water and oil was still entrained in the compressed air. The moral of the story is do not use compressed air that may contain any oil or moisture to clean, or in your case, blast clean aluminum.

I find it difficult to believe the aluminum, unless it was severely corroded, would allow oil to pass through the wall of the oil tank. Corrosion to that extent should be obvious if both surfaces are accessible for visual examination. If you have any concerns, apply dye penetrant (red liquid penetrant) to the opposite side and see if it wicks through after a couple of hours. If the aluminum is sound, it should be impervious to the oil. Were that not the case, we would have to purchase our beer and soft drinks in bottles only. After all, the thickness of those cans is only a few thousandths inch thick and rarely have I opened an empty can. :)

Best regards - Al

Valoman, is it possible that the surfaces that showed the problem had an oily residue on them before sandblasting that was driven into the surface, rather than being removed, by the blasting process? If so, heat could cause the oil to become visable on the surface again.