whisky barrels for plants

How can I treat a half-whisky barrel, to be used for plants, so the barrel will not self destruct in a couple of years? Is there an inexpensive, non-toxic oil or preservative that would help?
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Are these used barrels from actual wineries, or the kind you buy new at garden centers and big box stores?
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If you purchase the ones from a nursery, home improvement store, etc, no worries. My dad has had one for almost 20 years and it's fine.

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