It should be noted (IMO) that some finishes can be dissolved by acids (vinegar) and alcohol. For me, this is not an issue with Mineral Oil or Shellac since both are edible (as far as we know). The problem with Walnut oil is that it can become rancid. Pure Tung or Linseed oil may also be okay, but I typically stay with Mineral oil, or a mixture of Mineral oil and Bees Wax. Again, this is my opinion only and I'm certainly no expert. I just know what I'm comfortable with my family ingesting so I apply the same mind set to things I make for others. Oh, and polyurethane may indeed be impervious to acids and alcohols, but anything that leaves a plastic-like coating can chip and I'm just not comfortable with it.