Certain aircraft fuel cap types are prone to admitting rainwater if the o-rings deteriorate a bit or the cap doesn't fit tightly. For example I once had some friends with a Cessna Cardinal (177B) that acquired copious amounts of water in both tanks every time we had a significant rain. It's also a problem when refueling in the rain. It's less of a problem flying in the rain because the low pressure over the top of the wing and movement through the air actually can suck stuff out of the wing. This has been illustrated multiple times by guys leaving a fuel cap off after refueling only to have fuel sucked out of the tank(s) through the filler port.
That said, in my experience the business of condensation forming inside a tank isn't a given. I leave my tanks (2 @ 20 gal, rigid aluminum) as they are after a flight, with lots of air inside. For 5 of its 18 years it was tied down outside, the rest in hangars. In all that time the total amount of water I've ever seen checking the sump and tank drains wouldn't fill a shot glass. The plane does have a more elaborate venting mechanism than commonly found, so this might be a factor.
As far as my ground vehicles (bikes, cars, trucks) I don't do anything to deal with the "evils" of ethanol and they all run just fine. My infrequently used devices (generator weed-whacker, chain saw) do get a dose of Stabil in their fuel cans, but that's it.
Last edited by Rich Doyle; 07-11-2020 at 10:22 AM.