I frequently get dark stains on my white plastic panels that don't easily wash off. I mostly get them around the lower corner of my seat from water drips. I have been cleaning them with Goo Gone lately, and they wipe right off. I got a spray bottle of Goo Gone and wipe the plastics down with it before washing my motorcycle now, and it really works great at cleaning up the plastic panels. If you have some stubborn spots on your plastic, you may want to give it a try.