By law, above-ground gasoline tanks are ALWAYS painted white or silver. This minimizes solar heating and thus minimizes day/night breathing emissions. I'm a tank inspector from way back. I had a consulting business, and I've been around, over and in thousands of tanks, from 500-10,000,000 gallons.
E10 loves to absorb water from the atmosphere. If the water content reaches about 0.5% (more in summer, less in winter), the water will separate, taking the alcohol with it and creating a 15%-by-volume layer that won't burn. Phase separation, as this is called, is very unusual because absorption from the air is a very slow process, taking months. The usual cause, in fact, is a leaking filler cap or vent, not absorption. It's a slow process, taking months to become important. But prudence requires closing the vent whenever the engine is not in use. Spray can sneak in through the vent.
Water absorption does NOT need to reach phase-separating levels to cause problems. At just a few hundred ppm, sludge generation increases. The gasoline becomes weakly conductive and galvanic corrosion blooms in the carb bowl (brass and aluminum are an active pair). Gasoline also evaporates, and the loss of the most volatile portions can make starting more difficult, and increase gum formation. Better to keep it sealed.
But a sealed tank builds pressure. Over time, UV damages the tank and the tank becomes weak. It would be nice to keep it out of the sun. Some people like Sunbrella covers. But on my boat, it would just get grungy with spilled oil, gas, lichens, and algae.
Paint is simpler. Since the tank is mostly covered by the seat, there was no need to paint the whole thing. If it was, I might paint the whole thing, leaving only the code stamp exposed (not on a seam).
Simple. The tank is protected from UV and stays cool. It's been seven years, 365 days per year, and the paint is still there.
