The answer to this is in two parts.
First part is that oil and water don't mix... period.
Ethanol is water based and petrol is oil based.. have a real good think about that because that's what these pricks are mixing together and forcing us to fill our tanks with.
So how do the oil companies achieve the blend?
They use a catalyst and from past experience with speedway fuel (methanol)
the catalyst they use is....Acetone... at 6% and that's the bugger that causes the issues with plastics, diaphrams, plastic and glass fiber fuel tanks.
The water is just a secondary issue when fuel evaporates in carby bowls over time and leaves water (and other unburnable additives) behind.
That's why injection systems are far less affected as they are sealed and under residual pressure at rest so the fuel can't evaporate in the system leaving the water and crap behind.
The tank is another matter but keeping it full helps a lot.
I have 4 carby bikes that get used in rotation about once a month so I turn the fuel taps off and run the carbys dry after a run and add a tiny amount of two stroke oil to the fuel (0.1%) to help coat the inside surfaces, floats, jets and needles with a protective film when they are not running.
Fwiw plastic fuel tanks are made from these five different materials: high density polyethylene (HDPE), polypropylene (PP), regrind plastic (recycled polyethylene), a plastic adhesive or ethyl vinyl alcohol (EVOH) and all of these absorb water at various rates.
I make plastic components that live in automotive cooling systems and they have to have a growth allowance built in to compensate for their size increase over time due to water absorption.
It's a pity motorcycle designers were ignorant of this fact when they made plastic tanks for bikes and fitted close fitting covers.
2007 Cagiva Mito.
1980 and 1978 Ultra lightweight Yamaha SR500 Cafe Racers
1967 500cc Ultra lightweight Triumph Daytona Cafe Racer