We actually do use a very diluted bleach solution on PAINTED decks that have extensive mildew. The paint does not allow the wood to absorb the bleach, so it is safe to use in very diluted strength. We never use bleach on raw, untreated wood. Bleach will affect the wood’s cell structure affecting future stain, sealer, and most importantly paint absorption/penetration leading to premature greying of decks, fences, or much worse product failure (peeling).