If the metal tube is strong enough to hold the wood when it expands from moisture, the wood will be compressed. When the wood dries out, it will no longer fit the metal tube - it will be loose in the tube.
If your design depends on the wood being tight in the tube, you've got a problem. No finish that I'm aware of will prevent the wood from absorbing moisture when the unit gets wet (when it rains on it, for example).
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