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.

The problem that needed repeated fixing on my M20J is getting the FBO minimum
wage guy to put the damned caps on correctly! I've had that problem in too many
places, esp on fairly long XC flights where you land in the rain, tell them to
top off the tanks, go to your hotel, work the next day, go out to the airplane
in the rain, drain and drain and drain the damned tanks because -- well, you
know the story. Have you noticed, as I just did, that drain is D RAIN? As in
damned?