What causes mildew to return every time we clean it off of porch & house overhang?
It is painted wood.
Lack of drying, or sunlight are the leading causes of mildew return. Do you have alot of trees close to your home ? Marine paints usually resist growth better. Are you useing bleach with the washing ?
It's most likely the lack of sunlight that's the issue. Mold will form on any surface not just the painted wood if the environment is right. If you're using bleach to eliminate the issue you could be making the issue worse. During application the bleach can evaporate into the air and the water gets down into the wood which is the perfect breeding ground for more mold. You might want to consider Wet & Forget it's a moss, mold and mildew remover that doesn't contain harsh chemicals that can eat away at your painted surface meaning you'll have to paint sooner which is never fun. You just apply the product and you don't have to use a power washer or scrub the area the wind and rain will slowly eliminate the issue. If you see the mold starting to come back in a few years just apply again as needed.