To get an idea as to how dirty this vinyl tonneau was before I cleaned it, here's a picture that shows how dirty and stained the paint was.
Here's a section I've taped off on the cowel because me and one of our forum members already buffed out the hood when he stopped by to test out polishers.
On the left I've clayed the paint and then machine polished it twice and then applied a coat of wax. You can clearly see the left side is now a brighter color of yellow and the right hand side has what appears to be grayish film over the paint.
Here's the same picture above with graphics....
Think about it... the vinyl tonneau cover is exposed to the same dirt the paint is exposed to so it only makes sense that if the paint is dirty with embedded stains.. so is everything else on the car...