Finn here:
The saunas are let to dry properly. The wood is impregnated with parafin oil regularly to ensure that nothing goes in to the grain of the wood. Then you also wash the sauna after use and properly regularly and you let it dry and keep it dry when not in use.
However here is the thing: They do rot away slowly, you have to do maintenance of them. When you see rot or decay, you have to take action and remove it to preserve the overall structure.
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