If the mildew formed underneath the stain, it will require a more complex treatment. You will first need to remove the stain, then use a sodium percarbonate cleaner to eliminate any remaining mildew.
Keep in mind that, when it comes to keeping your home in pristine shape, preventive steps are key. Babcock advises starting with a quality stain with mildewicides that will prevent moisture from seeping into the wood. For timbers at a higher risk of accumulating mildew, use a mildewicide additive in the finish as well.
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Be sure to perform maintenance checks in the spring and fall to test and clean the wood. If you find the timbers soaking up water, a touch-up may be necessary. Also remove any dirt, pollen, bird droppings or sap that may have accumulated on the timbers, as mildew spores can easily attach to these contaminants, says Babcock.
Finally, check for any cracks in the timbers. “Anything larger than an inch should be sealed with a high-quality wood-specific caulk,” she says. “Anything smaller than that should be thoroughly soaked with the stain or finish. These little cracks can be highways for both moisture and insects, and keeping them sealed will prevent problems, as well as prolong the life of the stain or finish.”