It is common for sauna wood to get stained from our body's natural oils & sweat. If the sauna is near a pool, we have the effects of chlorine as well.
Now, when the rooms wall materials and benches get beyond a good scrubbing with soap & water, it's time to start sanding.
Get a fine grit sandpaper and sand the wall and any other areas that are not coming clean. After you have exerted all that energy into renewing the look of the room, how can we slow this down from happening again.
First, Backrests are important. You mount the backrest which protects the wall and also provides comfort to the patron. Then you only have a dirty backrest to deal with.
Click here for Backrests
Second, use an industrial strength Sauna Cleaner that is created for the environment and safe to use.
Click here for Sauna Cleaner