I had a PT deck that lasted 25 years without anything more than a couple
power-washes (mid-west US, so it saw all kinds of weather), and would have
lasted a lot longer but we took it out to add a room. The need to stain or
treat is really more for appearance. Untreated it will weather to a gray
color, however stained will look good - but take regular maintenance to
maintain that appearance.
So I'd say to increase the longevity, any treatment is questionable in its
value. To maintain appearance it has value - if you are willing to do the
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