As a builder, I prefer to either paint it with an exterior trim paint, or
leave it totally unfinished down here on the Gulf Coast.
All natural finishes applied to WRC, like stains with and without topcoat,
that we have attempted to use have all turned unsatisfactory after two or
three years, without exception.
Just my experience ... YMMV.
I would apply a finsih, Just to help repel water, it will last longer.
i would use a polyerene made for outdoor use. I have some information
on outdoor projects on my web site.
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