Camco Camper/RV Pro-Strength Black Streak Remover, 32oz (41008)

I used this on a stubborn black streak on the seam of my Casita Spirit Deluxe (fiberglass camper). I've been trying to get rid of this black streak for a long time. My first attempt was with following the directions on the bottle, wet and leave spray on for 30 seconds and scrubbed with a plastic bristle brush. That didn't cut it, no different from anything else I tried. On my 2nd attempt, I sprayed it straight onto the black streak without water first. I let it sit a while and lightly scrubbed with a wire brush. I could see the streaks were dissolving and hit it again and same process. The streaks are gone!!!