Mildew on wood is quite common, especially where wood is subject to warmth, moisture, and dim light. Mildew thrives under these conditions and can grow on wood outdoors or indoors. Treating indoor wood mildew may be more involved, but can be remedied nonetheless.
Step 1 - Remove Paint and Varnish
If your indoor wood has mildew growing under the paint or varnish, you will need to remove it. Sand off the varnish or paint using sandpaper. Paint stripper may be needed on wood with thick paint or varnish.
Step 2 - Pick Chlorine or Oxygen Bleach
Chlorine bleach is very effective at killing molds and mildews. Unfortunately, it has harmful effects. Chlorine bleach can actually break down the lignin that holds wood together. This is similar to cloth fibers. Chlorine bleach also gives off toxic fumes. Using an oxygen bleach is a better and safer alternative. Oxygen bleach will not harm wood fibers. It is also non-toxic and safe for the environment. You can purchase oxygen bleach as a liquid or powder. When using any type of bleach, never mix it with ammonia, as this can create a dangerous toxic gas.
Step 3 - Mix the Bleach
To mix oxygen bleach yourself, check to see if your spray bottle has a measurement guide on the side. If not, most spray bottles are either 24 or 32 fluid ounces. You can use a measuring cup to determine the amount the spray bottle holds. Using the oxygen bleach, mix 6 to 8 ounces per gallon of hot water (or 2 ounces per quart). Hot water activates the sodium percarbonate in the oxygen bleach. Now add a quarter cup of liquid dishwasher detergent (ammonia-free). Shake the mixture well to create emulsion and maximize its effectiveness.
Step 4 - Spray the Wood
Spray the bleach mixture onto the wood. Allow it to sit for 10 to 15 minutes. This will give it time to soak in and attack any mildew deep in the wood's crevices and fibers.
Step 5 - Clean the Wood
For shallow surface mildew, use a sponge and bucket of water to clean the wood. Wipe the surface thoroughly to remove mildew and oxygen bleach. If the mildew is deep in the wood's grain, you may need to use a brush and lightly scrub. When finished, clean the wood with a sponge and water.
Step 6 - Repeat as Necessary
When the wood is dry, inspect it for any remaining mildew. If mildew is still present, reapply the oxygen bleach.
Step 7 - Prevention
Remember that mildew thrives in warm, moist, and shady environments. Reducing the relative humidity indoors can prevent the return of mildew. Running the air conditioner will reduce temperature and using a dehumidifier reduces moisture levels. While ultraviolet light is not likely to kill established mold, it will help to prevent new mildew from forming. Ultraviolet lamps are available for purchase or you can simply use sunlight to deter future mildew growth.