I have noticed light black streaks, that appear in the area of a white concrete (mostly slanted driveways) after I have just cleaned it, It appears to be the width of my side walk cleaner. I have tried to go back over them, but they seem to stay. I have tried new noozles when I started to notice them, and tried to see if the hot water was the cause, and used only cold. but they are still there. and they seem to only appear on slanted surfaces.
Brian
Brian