at the risk of sounding dim, my petrol tank is rarely full, and i'd say it has rarely been totally full for the past 16 years, and it's still sound, not doubting just confused.
i suppose if he tank was full there'd be no air in the tank, therefore water vapour couldn't condense and send the petrol wappy.
my head hurts, to be safe i'll pack my tank with waxoil.