There is a third option.
Seal the vent when not flying.
The vent in my fuel tank is a -4 size AN bulkhead elbow.
I screw a cap on this fitting when not running the engine.
When the cap is removed there is usually a hissing sound indicating a vacuum or pressure in the tank. This also prevents fuel vapors from escaping which can be a hazard themselves.
Jim Maher

Ernest Christley <echristl@cisco.com> wrote:

The end result is a pumping action. A very insidious and unavoidable
 one. It doesn't matter what they tank is made of. A full day's heat
 soak, followed by a full night's cooling will make the pump work. The
 only way to stop is to:
1)Live in a place without water vapor problems. Either a very dry
 environment, or one so cold that all the water sits frozen on the ground.
2)Seal the tank perfectly completely. At which point it won't work well
 as a vehicular fuel tank, but at least won't have to worry about water
 in the fuel.

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