Despite no visible water when sumping, I have had ice crystals form in the Jetprop fuel system on longer flights, triggering the fuel filter bypass system, and so I now add Prist when refuelling for trips of 3 hours or more.
A lot of condensation forms when the wings get cold soaked with the warmer Jet fuel.
Prist also acts as a fungicide, so I also use it if I know in advance I will be parked up for an extended period of time.
In the US, most airside supply of Jet fuel is already premixed with Prist.
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Well, it works thousand times but only needs one time to bring you in serious trouble. Problem is not the fuel but the water in the fuel.
I used avgas up to minus 40s OAT and never had an icing problem. May depend on your fuel filter arrangement but in the PA46 it was fine.
Also, cold temperature protection is really only required for the kind of temperature normally encountered when flying very high, at which point car fuels with their higher vapour pressure can become problematic.
It is done IAW the aircraft mfg’s limitations. In the section concerning fuel icing precautions, Socata specify max 2% alcohol in the fuel. So this small amount is permitted. Also 100LL is permitted to contain up to 2% (IIRC) alcohol so in theory you might get that free of charge 
“For avgas, one can use IPA (isopropyl alcohol)…..”
Why do not use Super Plus ? It contains 5 percent ethanol. Isn`t it a contradiction to avoid ethanol due to possible damage of seals while putting in isopropyl alcohol in the fuel tanks? Does IPA not damage the seals ? because it has another OH-ending ?
One could add stuff to absorb water in fuel, if operating in wet/humid conditions. This absorbs a certain amount of water and helps to protect against fuel freezing on the way to the engine (BA038 type of thing).
For avgas, one can use IPA (isopropyl alcohol) or PRIST/EDME.
IPA is cheap, non-hazardous (so long as you don’t set fire to it 
) but a fair amount is needed – around 1%. This is not a big deal in most cases but for a long trip it is quite a lot of liquid to carry. I use substantial HDPE bottles from a laboratory supplier.
PRIST is carcinogenic/toxic but has the great advantage of being needed in only a very small % – of the order of 0.1% I think. Ferry pilots use it.
It is important to have the filler cap seals in top condition. I am certain that the vast majority of cases where lots of water is drained from the drains are caused by duff filler cap seals – not by condensation inside the tank as we are taught! PPL schools in particular are notorious for not spending money on stuff like that. I used to drain out masses from the PA38s I started my PPL in, and in one case drained out about 1 litre after a rainy night.
Nothing wrong with flying in a little rain. If a refueller then it is their problem (has nothing to do with mandatory handling). Umbrella really is the only solution.
Water can take a while to get to drains so if you need to fly quickly, best to keep it out rather than try to take it out later.
I don’t use prist. Horrible stuff.
I use Jetfuel and fly high. Jetfuel is save to -47C but water will freeze and may then clog the fuel lines. I add Prist so I’m save with the normal condensing water. But last weekend I had to refuel in Salzburg during intense rain and used a cloth which was getting wet quickly. So I thought I should buy something for that…
Excuse me for playing the devil’s lawyer, but what is wrong about a few drops of water getting in? Why do we have drains, and draining procedures?
And anyway, who thinks of going flying when the rain pours down (cats and dogs was the proper term, ISTR)?
Except those who fly from aerodromes with mandatory handling, but then the umbrella is the mandatory handler’s issue, isn’t it?
[[edit: dublinpilot seems to think along the same lines]]
