How to clean a sauna is mostly a question of what not to do, because the interior timber is bare by design and most household cleaning products are exactly wrong for it. Done right it is ten minutes a week and an hour once a year, and the cabin looks and smells better after five years than after one.
The weekly routine
Wipe the benches and backrests with a damp cloth after the last session while the cabin is still warm, and leave the door open so everything dries. Lift removable benches occasionally and sweep or vacuum under them. That is genuinely most of it: a cabin that dries properly between uses does not develop problems.
What never to use
No bleach, no ammonia, no pine cleaners, no varnish, no oil soap and nothing scented. Bare timber absorbs what you put on it and then bakes it at temperature, which is unpleasant and can be worse than unpleasant. Water, a soft brush and a mild non chlorine cleaner formulated for sauna use are the whole list.
The annual sand
Once a year, or whenever the benches look grey and feel rough, sand lightly with a fine grit along the grain. It removes surface staining and sweat marks, restores the color and brings back the scent in cedar. Vacuum the dust afterwards. This is the single job that keeps a cabin looking new and almost nobody does it.
Stains, smells and the stones
Sweat stains sand out. A musty smell means the cabin is not drying, so check ventilation and the floor first rather than reaching for a cleaner. Rinse and restack the heater stones annually and replace crumbled ones. If you use towels on benches as a habit, most of this section becomes theoretical.
Questions people ask about how to clean a sauna
Can I use bleach in a sauna?
No. Bare timber absorbs it and then bakes it. Use water, a soft brush and, if you need one, a cleaner made for sauna use.
How do I get rid of a musty smell?
Fix the drying, not the smell. A cabin that airs properly after sessions does not go musty. Check the ventilation path and under the benches.
How often should benches be sanded?
About once a year, or when they look grey and feel rough. Fine grit along the grain, then vacuum the dust.