Sauna wood does not rot, warp or go grey because it got wet. It does that because nobody ever did the twenty minutes of maintenance that keeps it from happening. Here is the whole job, twice a year.
Why benches go grey
Grey timber is surface degradation: sweat salts, body oils and repeated wet-dry cycles breaking down the top fibres of the wood. It is cosmetic at first and structural eventually. It is also almost entirely preventable, and largely reversible if you catch it in the first year or two.
Never varnish a sauna bench
This is the mistake that ruins benches permanently. Varnish, lacquer and standard wood stains form a film on the surface. In a sauna that film gets hot, traps moisture underneath, blisters, and becomes unbearably hot to sit on. You then have to sand the whole thing back to bare wood.
What you want is paraffin-based sauna oil: it soaks into the fibres rather than sitting on top, does not form a film, and does not get hot to the touch. It is the only thing that belongs on a bench.
The twice-yearly routine
Let the sauna cool completely and dry out fully — at least a day after the last session. Sweep and vacuum the room. Wash the benches with a mild sauna cleaner and a long-handled brush, working along the grain, then wipe down and let everything dry for several hours.
If the wood is grey or rough, sand lightly with fine paper (120–150 grit) along the grain until you reach clean timber. Vacuum the dust properly, including between the slats.
Then apply paraffin oil thinly with a cloth or sponge, along the grain, and wipe off anything that has not soaked in after fifteen minutes. Leave it to cure for a day before heating the room. A care set bundles the oil, cleaner, sponge and paper so you are not hunting for one missing item halfway through.
What to do after every session
Almost nothing, which is the point. Leave the door open so the room dries out. Run the ventilation if you have it. Take wet towels out rather than leaving them draped over a bench. If someone sat directly on the wood, wipe that spot.
Sitting on a towel or a felt seat pad is the single most effective preventive measure there is — it keeps sweat off the timber entirely, which is what causes the greying in the first place.
Duckboards and floors
The floor takes more water than anything else. A wooden duckboard keeps your feet off cold tile and, more importantly, lets the floor underneath dry. Lift it and dry the floor properly every few months, or you will find the problem you were avoiding growing quietly underneath.
Heater stones are not permanent
Stones crack and crumble with heat cycling, and crumbled stone restricts airflow through the heater — which makes löyly weaker and the element work harder. Take them out once a year, discard anything cracked or flaking, rinse the rest, and repack them loosely so air can move. Most heaters want a full restack every one to two years.
Wool and felt
Never machine wash a felt hat. It will shrink into a small hard bowl. Rinse in cool water, press the water out without wringing, reshape it while damp and dry it flat away from heat. Linen and cotton towels are the opposite — wash them hot and often, and skip the fabric softener, which coats the fibres and destroys absorbency.
We do this to our own sauna twice a year, which is how we know which oils smell fine and which ones make the first session afterwards unpleasant.