Outdoor wood finishes

[Greenie]

Even the best spar urethane will crack and peel in two years - I've come to accept that. However wood surfaces that are painted may last 10 plus years.
On Adirondack chairs built from white ash for example, unstained but finished in spar urethane after two summers out in the sunlight and the urethane is flaking off. I always thought the sunlight and varying temperatures expanded and contracted the wood while the finish didn't - but if I painted those Adirondack chairs I'm fairly certain the finish would last at least 5 times as long as clear Urethane.
Why is this????

[beenthere]

The UV from the sun breaks down the wood under the surface coating. Destroys the lignin and leaves only cellulose. The coating has nothing to adhere to when that happens.

Paint stops the UV from getting to the wood...