Well if it is for longer term storage, you can add dessicant cartriges in place of the tank vents, and restrict the tanks "breathing" when not actively transfering fuel. This will keep the tank from injesting moist air which condenses on the inner skin. the dessicant canister connected to the tank will ultimately draw the moisture out of the air trapped in the tank. I put a rubber cap on the cannister port connected tothe atmosphere with 2 pinholes pushed thru it. that way if the tank pressure differs too much from atmospheric pressure due to heating or cooling, the tank can expell or draw air so as not to develop excessive pressure. You will have to open the vent up(or remove the rubber cap when you transfer fuel, but any air will be drawn thru the dessicant cartridge...