The only product that we carry that will treat both fungus and mold on wood is Bora-Care with Mold-Care. You can use this product on any raw wood or other cellulosic material (without paint, stain, finish, etc.).
View the BoraCare Mold & Fungi Bulletin for more information.
Tim-bor Professional can be used on raw, untreated wood to prevent wood destroying insect infestations and decay fungi if used as directed on the product label. As long as you wait until the application has completely dried to use the sauna, there should be no problem with treating the exterior. Do keep in mind however, that this product leaves a white powdery coating after applied.
If you are want the wood to look like its original state, you may consider using Bora-Care instead, as it dries clear. It is also advisable to apply a water sealant after applying a wood treatment for wood that is exposed to the elements, if the sauna is outdoors.
Tim-bor Professional needs to be applied only to raw wood surfaces in its bare state to be effective. If there is a stain, pain, or any type of sealant then the Timbor will not be able to penetrate thus making it ineffective. Any finish that has been applied to wood should be sanded off so that you can apply the Timbor for it to be effective.
Timbor does leave a white powdery film on wood surfaces so if you will be staining, painting or sealing the wood surface BoraCare would be a better option as this dries clear.
Per the product label, you can inject or dust Timbor at a rate of 0.5 oz per square foot when treating wall voids for insects such as carpenter ants. If you are mixing Timbor in water and creating a foam with a foaming solution and foamer, then you would use 1.5 oz of Timbor per gallon of water and use a high expansion foam. Keep in mind that drilling into and injecting a product like Timbor into all your wall voids is a tedious and labor-intensive application that is often best left to professionals. We have a great Carpenter Ant Treatment Guide here that may better assist you.
