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There is a product that I used on a galvanized gas tank from a 1952 Chriscraft that had rust inside the tank. It was a liquid plastic type substance that I poured into the tank (out of the boat and thoroughly cleaned) and rolled it over and over so the plastic coated the entire inside surface of the tank. Once it was completely coated, the extra was poured out and the coating set up. Worked well. No more tiny rust particles in the carb. I can't remember the brand name but I think I remember getting it at NAPA. It wasn't cheap but it worked. I used a couple of quarts for a 20 gal tank and probably poured 3/4 of a quart out once the tank was coated.