I have fenders that are in good condition except each one sems to have collected a coating of black tar on the upper end. I have tried removing it with soap and a stiff brush but it is very difficult to remove, and ends up marking the hull with black streaks. Have others had a similar experience with fenders- it appears that something in the air is collecting on the top and it is similar to tar or even asphalt. What have you used to get this gunk off or is it time to throw the old fenders away and get new ones. Except for the tar on top they are in good shape. Thanks!