What type of sealant should I use in a square foot garden?

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Question by Marshall: What type of sealant should I use in a square foot garden?
I am building a 2X8 foot raised square foot (vegetable) garden. Using plywood and other random wood. Someone suggested I seal it with polyurethane or exterior house paint so that it won’t rot, but won’t that be bad for my vegetables?

Thanks.

What do you think? Answer below!