My dh is working in the conservatory on his bike with it turned upside down and was spinning the back wheel. I open the door and it caught the tyre leaving a dark stain on the White PVC . Have tried everything and got some of it off but a stubborn strip remains...any ideas? (have tried bleach, normal kitchen cleaners, toothpaste, baking powder...not at same time! Most success was bleach but it won't get rid of last bit)