From one of my first trips out, I got a scum line that I could not remove with anything. I tried every known cleaner I could find. Nothing would touch it. I finally resorted to wet sanding one side of the boat which took 600 grit to even touch it. It took hours. I was hopeful to find a different solution for the other side so I went to the store and found this Hull Cleaner. It took the scum line right off with very minimal scrubbing. More like wiping. I'm a believer.