I suffered through this issue on my first NSX. The problem is all of the cleaners just push dirt into the gouges and scuffs in the plastic. So you end up with a clean white surface full of black lines and scuffs. It's like doing a panel wash on a plastic model. The only way to really "clean" the surface is to sand it. It's just like paint- start with a coarse grit that is large enough to remove the deepest scratches. Then work through finer and finer grits until you are at something like 1,200. I never got to do this before I sold the car, but given the level of effort and time involved, I resolved to either just buy new sills next time, or get a black interior. 317 has a black interior.