I suppose the water becomes a resident when it has stayed in the same place for more than 6 months. 
And there is no obvious mechanism for substantially or completely replacing the air in the tank with fresh humid air so it can re-condense over and over again.
I think that is essentially correct, but it’s a bit more complicated – you haven’t considered the fuel vapour.
According to BP
and Shell the (saturated) vapour pressure of avgas is 380-490 hPa at 38C (i.e. its Reid Vapour Pressure is 38-49, 1kPa = 10hPa) and this table
at the bottom of this Transport Canada page suggests the fuel vapour pressure of RVP50 drops to about 140 hPa at 0C.
So, assuming the tank temperature lags the air temperature, on a 40C hot and humid day, as the fuel tank warms up the fuel vapour will expand to fill about half the tank volume (490/1013), pumping out an equivalent volume of air and water vapour.
If it freezes overnight, the fuel vapour will slowly contract to about 14% of the tank volume, pumping in the equivalent amount of air and water vapour.
So I would expect the air in the tank to be replaced over time, but typically it is cold dry air that comes in each time, not hot humid air.
I would be very mindful of the water drains when flying diesel engined aircraft.
I am, also because Thielert people warned me that the fuel pump(s) really hate water. I have also heard that you can count on getting water along with with jet fuel. Assuming that it is not Perrier, could that be the reason why it is cheaper than Avgas? 
I must have been extremely lucky. No water at all in 8 years flying Diesels.
Now flying a Rotax, i do get some water. Matter of a few drops only. Fuel caps are fine, aircraft hangared, no flying through rain. So it comes from the fuel, in my case Euro95. I’m going to do some experimenting, filling one tank with a funnel that filters out water and the other without, and seeing whether Euro98 makes a difference.
I’m not too worried about a few drops though The fuel pick up point is higher than the drain point, and then there is the gascolator. Very unlikely that a significant qty of water gets to the the carbs i guess.
Wondering how this works with cars though. No manual draining ever and would this not lead to an accumulation of water in the tank over time? Maybe the tank or engine has a more sophisticated water separator and which does the draining continously?
aart wrote:
Wondering how this works with cars though.
Good question. I am no expert but on all automotive petrol engines of recent times, unless the amount of water is enough to displace most of the fuel being pumped, a quantity of water pumped into the induction together with fuel will cause a lean mixture which will be immediately detected by the exhaust sensor and result in an increased amount of fuel being injected, correcting the problem and becoming unnoticed. I think it would only be noticeable at the high-end of the FF range (where all flow capacity of the system is exhausted and there is little room to correct) which is little used in normal driving anyway.
Most large aircraft have fuel tank recirculation systems to ensure water accumulation is minimized as well as melted, but still require periodic draining.
On a light aircraft, however, even if you had an electronic fuel injection, you would notice a lack of power and a lean mixture at take-off, as a minimum. Perhaps in cruise with lean mixture it would be able to compensate like cars. On more typical aircraft with mechanical injection, or carburetor, where mixture could be manually adjusted to compensate, at least in cruise, the problem would be that the ratio of water-to-fuel would not be constant and you would not be able to manually correct so quickly, resulting in a rough running engine if you are lucky, or a stoppage if not. If that happens in flight at altitude and the aircraft is injected, and switching tanks does not fix it, I would just keep on pumping fuel/water into the engine using available pumps in an effort to get rid of all the water and try to recover the fuel supply, but, question for the group, how would you be able to tell that is your actual problem?
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Condensation can DEFINITELY form inside all metal fuel tanks.
bq. Quote
I agree but it takes the right conditions to form. The risk factors here include leaving an almost empty sun heated tank outside where potencially humid air inside the tank will condense to water droplets on large temperature drops overnight and collect at the bottom of the tank as a water pool. Parking inside a hangar with more stabile ambient atmosphere should take care of the problem and if parking outside make sure the tanks are full.
A leaking fuel tank filler cap in rain can contribute to very large amounts of water, but water should not enter in flight as the low pressure on top of the wing sucks out from the tank if any leakage. A leaking filler cap with a full tank will create a nice fuelspray over the wing.
I’ve operated the same PA-28 and C-172 for a good many years (and 2500 Hrs flying) and never seen any water in the fuel, ever. During that time I’ve had the actual caps replaced at least twice due to wear and weak springs, and the seals numerous times. Yet fuel in water stories persist with other aircraft.
The PA-28 and C-172 caps sit proud of the wing. Other types have them recessed for aerodynamic reasons and can pool water. Any seal will leak over time, so maybe that’s the true explanation?
Ref fuel streaming from the wing, not on the PA-28 with full tanks. Don’t ask how I know that!
In 2000, the first year we owned our current Jodel DR1050, the Syndicate as usual went for the weekend to the Glenforsa Fly-in. It rained. We were using 4* leaded mogas. The front tank was O.K. but the rear tank, whose filler is on the vertical side of the fuselage, drained yellow blobs in the fuel.
I took a sample to an ex-oil industry chemist, and he suggested some kind of alcohol. After several months in the sealed tube, at room temperature, the blobs disappeared.
Avgas can hold about 0.1% water; anything beyond that condenses out.
I am pretty sure all cases of significant amounts of water in fuel is either due to perished filler cap seals or due to some unusual storage situations where humid air condenses and gets forced into the tank.
@DavidS would always come in with excellent analysis
Nothing to add, except we recently started finding significant water in the RH fuel tank (only) of our group C172, after years of finding nothing. (In a C172 the RH tank is the one furthest from the atmosphere, as the fuel vent is into the LH tank).
We could only think of three explanations:
1) The UK weather has changed (so why RH only?)
2) The physical chemistry of air and water has changed (exciting, if true)
3) We might have a leaking filler cap.
For now, we are going with 3 :-)
