Well, while more are bare, still a fair number of them you see are stained and polyurethaned.
Personally though if I were going to seal some, I think I'd go for for West marine varnish for sheer durability. BUT... all that I've ever seen (and frankly I've only ever used it for boats) was high-gloss finish. For this application you'd probably want a satin finish (just in the off chance that you caught an unwanted reflection), but I don't think you can usually get marine varnish in anything except gloss. I could be wrong about that.
Barring that, I'd probably go with satin polyurethane. Thompson's is a good product, but it's something that you'd have to re-apply periodically. I'd rather do it once and be done with it.
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