Simple detailing question

[Jimi]

I have a 2000 boxster S, triple black. In prepping for winter storage I've clayed the whole car (my first time using clay). Now, I've noticed the area below the doors than runs the length of the car from wheel well to wheel well is "different". It's rough. I did try to clay a bit of it as a test and realize it's not just paint. So, what is it? How do you treat it? It doesn't seem to be rubber, either. I don't know what it is or how to treat it. Is there anything special about it? I've treated all the vinyl/rubber stuff (inside and out), the bumperettes, and the plastic areas below the front bumper and the splash guards with 303 protectant.

Just not sure what to do with the "sill" area below the doors. My claying sure took off a lot of junk on the lower part of the side panels, so I'm sure this sill area is pretty gunky and not something normal washing takes care of.

Thanks, Jim