A sealed fuel container will only have the usual varnish deposition issues since no water vapor can get into contact with the fuel. Ethanol will phase separate if there is water vapor available since the ethanol enhances water dissolution into the gas. When the water content reaches an appropriate level the fuel separates into a water rich layer with most of the ethanol and a gas rich layer depleted in ethanol. A vented container guarantees that the fuel will see water vapor. Just think back to your basic organic chemistry classes in college.