Mildew on the outside of your house may look like black or green streaking on the shaded side of your house. If the area you suspect is on a sunny side of the house it is doubtful it is mildew and it may just be dirty. Prior to painting mildew should be treated with a mild bleaching solution and power washed off. You can purchase ready made products from you local Home depot or Hardware store, or your can make a 15% solution of bleach with water your self. Apply the bleaching solution with a garden sprayer and leave it for 5 to 10 minutes before your powerwash it off. If your mildew is severe I recommend you do the same process a few times to be thorough. If you do not properly treat the mildew areas before painting your paint job WILL fail. Call 619 464 4030 if you would like us to take a look at your project.