Things You Will Need
Depending on how bad the mildew stains are, one or more of the following: Liquid Ivory soap or another mild neutral ph detergent, 1/2 cup Lysol to a gallon of water. If the mildew stains are persistent, a mixture of chlorine bleach or ammonia in weak solutions with water or mildew products such as Tilex or X-14. NEVER MIX BLEACH AND AMMONIA TOGETHER.
If the wood surface is wicker, you can usually clean and kill the mildew with a simple solution of bleach and water (1 cup bleach to 1 gallon water is usually strong enough) or 1/2 cup Lysol to a gallon of water. Spray the solution on the wicker, let sit for a few seconds and then rinse thoroughly with warm water. Allow to dry completely. You can then paint with a paint containing a mildewcide.
For unfinished wood, brush off any loose mildew and clean with a non ammonia detergent such as liquid Ivory soap (1/4 cup soap to 1 gallon water). If the stains are still present, you can use the bleach solution above but be aware that bleach can turn wood white in the areas cleaned. Use it sparingly and rinse the cleaned area quickly. You can also try a diluted solution of ammonia and water mixed at 1 cup ammonia to one gallon of water. If you use the ammonia, if at all possible do it outdoors or in a well ventilated area. Again rinse thoroughly. Let the wood dry completely before using or applying a finish.
Finished wood is a whole different set of problems. If the finish is gone, use the ammonia solution in Step Two to rub the area down. If the wood is finished with shellac, chances are the mildew is growing under and in the finish. You can test to see if it is shellac by rubbing a small area with denatured alcohol in an inconspicuous spot. If the denatured alcohol removes the finish, it is indeed shellac. If it doesn't remove the finish, you can use the bleach solution as in Step One above. If it is shellac, it is much more difficult to save the finish and you may have to strip the finish completely and then kill the mildew and refinish the entire piece. If it is in a small area, you can try applying a small amount of denatured alcohol soaked into a fine grade steel wool then rubbing the area lightly. This will remove the mold but it will also remove a layer of the finish. When this dries completely, you can smooth the area with very fine grade steel wool and then buff out with a furniture wax
Mold and mildew growing on wood stains and destroys the natural beauty of the wood. With proper care and cleaning, most woods can be brought back to their original condition.
Tips & Warnings
Start with the mildest soap solutions first to see if they will successfully kill the mildew and remove the stains before using the stronger solutions. Wear gloves and safety goggles when working with the stronger solutions to protect your skin and eyes.
Antique furniture that is finished with shellac is sometimes better left up to a professional.
Bleach, ammonia, and other chemicals used to clean mold and mildew can be poison and the vapors can be toxic to individuals with breathing problems. Some individuals also may have allergies that can be aggravated by the mold and mildew itself.
NEVER MIX BLEACH AND AMMONIA