Both of these wood species contain large amounts of natural chemicals called tannins, which give characteristic color and odor and afford natural protection from wood rot. A high concentration of these components on the surface prevents strong penetration of a finish. Often, bleach is necessary because it is the only known product that kills mold spores. At Flood, our tests show that when bleach solutions are left on wood no longer than 15 minutes and are rinsed thoroughly, they are effective in removing mold and mildew. However, bleach does harm plants and shrubs, and can cause damage if inadvertently left on the wood too long or not thoroughly rinsed, and should be used only when necessary.
Flood recommends spot cleaning mold and mildew with bleach and water, followed by thorough rinsing.
It is interesting to note that bleach may have a more positive effect when used with an acrylic finish. Wood preparation is more of a science than an art. For the longest-lasting, most durable finishes, good preparation is required. Several factors are involved when choosing a good preparation method. Marry the right products together.