With very minimal maintenance you can keep you sauna looking great. The following is a general guide.
Do not paint, varnish or stain the interior portions with any normal paint or varnish. The wood needs to breathe. Also artificial finishes on the wood can create a much hotter surface on the wood, creating the possibility of fumes from the wood treatment, and take away some of the "softness” of the heat and steam penetrating the wood.
The simplest method of sauna maintenance is to keep a hand brush near the sauna door. The last person out dips the brush in the water bucket, and does a quick scrubbing on the benches, walls, backrests, etc. This 30-second ritual will keep your sauna looking great indefinitely. This will prevent the need to use anything stronger than clan water on the wood portions.
After you are finished using the sauna, and your cleaning process is complete, prop the duckboards off the floor. Leave the sauna door open, to air it out completely. The heat remaining I the rocks and in the wood will dry the sauna completely, and even can dry the shower area down, if it is adjacent to the sauna room.
If you get some dirt or sweat stains developing (if #2 is missed a few times), again use a hand brush but use warm water with a mild detergent in it. To get the benches looking like new, you can lightly sand your benches about once per year. It will whiten them to a "like new” condition. If you should happen to have mold develop anywhere (on duckboards, for example), you can clean with bleach.
Depending on how often you use your sauna, occasionally wet-mop the floor, with a liquid deodorizing cleaner. With these simple maintenance tips your sauna will stay inviting, fresh smelling and enjoyable to be in.