Like anything outside, wood decks tend to build up mold, dirt, fungus and algae deposits. The best way to restore and clean your wood deck is by pressure washing after using a mildew, mold, and dirt removal solution. Most people make the mistake of letting the wood on their deck go unprotected which leads to mildew and mold buildup and eventual rot. When you can see mildew or anything green on your deck it seriously needs some attention.