The other day I encountered a bit of moisture during a cold day's flying of about 300 Nm from Natal to my farm in Stutterheim.
Temp was about 12 deg. Celsius and not much rain at all.
When I wheeled my Kitfox into the hangar at home, I noticed these wrinkles in the tops of and the first two panels outboard of the fuel tanks. (Both sides).
The next day, they were gone and covering was nice and tight.
Any ideas from the clever, non-farmers out there??