I have managed to hold on to my Deluxe Ground Plate all of these years. And yes, it looked worse than yours.  Sandpaper, how quaint, but alas, manual labor, ahh NO.

But I was inspired.  So I pulled it out, lifted the edges and put metal duct tape under it to protect the plastic.  Then, I used a wire brush wheel with my portable drill followed by the sandpaper wheel (in four directions).  It’s REAL SMOOTH!

For now, I have coated with oil to keep it from oxidizing until I can get out and get something better to coat it with.