Hello all. I haven’t been very active on here lately because I have been fairly busy however I just ran in to a bid that I am not sure about so thought I would get a 2nd opinion from some of the pros on here. This is actually a commercial building with a dozen or so windows that have streaks below the window sills where you usually see the black mildew streaks. These streaks are grayish in color so I assume that they are from the aluminum framed windows. Does anyone have any experience removing these? I think we have ran in to this on a couple brick homes and were not able to remove with house soap or sh. I am wondering if a light acid treatment might do the job. I will probably offer to go out for a small price to try a couple chemicals on one spot. I just want to make sure that I bring the right chems. Thanks ahead of time.