I use sheepskin mitts and have never had a problem with any marring at all, and I do own a black car. While mitts may trap dirt particles in the pile sea sponges are also very pourous. Therefore particles, especially sand and larger particles, can get trapped just as easily and cause more serious damage. I guess it is a metter of how well you care for your sponges or mitts. I typically use two rinse buckets each filled with 0.5oz of QEW in 1.5-2 gallons of water to help ensure the cleanliness of my mitts, grit guards also. This type of matter would solely depend on the user and their washing/rinsing techniques. I can guarantee that my neighbors, who use one bucket filled with dawn, can cause serious marring any finish even with the best sea sponge that you can find.