Do not paint, varnish or stain the interior portions of your sauna. The wood needs to breathe (absorb and slowly release heat and humidity). Also, artificial finishes make the wood surface much hotter, create the possibility of fumes from the wood treatment, and take away some of the “softness” of heat and steam penetrating the wood.
Door handles and floor boards are an exception to step #1. These two points of the sauna can get dirty easily. To make cleaning easier, you can treat the handles and floor boards with a good wood sealant or polyurethane finish.
The simplest method of sauna maintenance is to keep a hand brush in the sauna. The last person out dips the brush in the water bucket (plain water), and does a quick scrubbing of the benches, walls, backrests, etc. This 30 to 60 second ritual will keep your sauna looking great for years.
After you’re 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 in the rocks and in the wood should dry the sauna completely, and even can help dry down the shower area, if it is adjacent to the sauna room.
If you get some dirt or sweat stains developing (if #3 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 lighten them more to their original condition. If you should happen to have mold develop anywhere (on duckboards, for example), you can clean with warm water and bleach such as Chlorox or Hi-lex.
Depending on how often you use your sauna, occasionally wet-mop with a liquid deodorizing cleaner such as Lysol or Pine-Sol.
With these simple maintenance tips your sauna will stay inviting, fresh smelling, and enjoyable to be in. The main rule is: ENJOY