I think there is another problem here. I've owned my Mooney for quite
some time and always kept it outside. I've never, ever, gotten water in
my tanks. I always replace all the cap seals (2 seals per tank) at
annual and ensure the cap tension is correct. I also never park my
plane with more than 15 gals per side (so much for the wives tale about
empty tanks collecting water). Even at that, I never get water. If a
Mooney pilot is getting water in his tank, he has a problem that needs
to be fixed.
-Robert, CFI M20F owner.