condensation can only occur above the petrol but as it condenses it will eventually gather at the bottom of the tank once the fuel has taken on as much water as it can possibly absorb (yes modern fuel is hydroscopic) at this point you will not get any bang from your fuel, secret is to brim the tank during prolonged storage as this has the effect of reducing surface area only fillerneck and reduces the void to create condensate by heat variations. just my 2p worth.
oh Hi I am new round here lol