Sauna backrests are durable and therefore, should be easily cleaned with water and mild soap. It is an important part of sauna maintenance, that you habitually clean your sauna backrest, especially if you aren’t the only person using it.
If you are concerned about damaging your back rest, check the brand specific information for maintenance and care. The cleaning instructions should give you the exact information you need to clean your sauna backrest without causing any damage.