Recommendations for protective finish for "Mahogony" (Red Merranti) deck?

I'm refinishing a covered deck with tongue & groove 1x4 Red Merranti.
Looking for recommendations for protective finish? - What should I coat the underside (and tongue/grooves) with before installation? - What is the best natural finish for the topside after installation?
Thanks
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Look into "Penofin" not cheap but very good
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