How do I protect my outdoor wooden planters?

I have four wooden planters I want to use as tables on my deck, however, I want to protect the wood from the elements and slow the weathering process. I'm not sure what kind of wood they are but the description said "teak finish". What product should I use to protect the wood? ( I plan on topping each with a thick piece of glass.)
q how do i protect my outdoor wooden planters
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