There is some kind of non-slip coating that is common on mice, keyboards, remote controls, etc. After a few years it starts to get gummy and sticky, and very unpleasant to touch.

I have had a lot of success using 91% isopropyl alcohol to remove this gunk. I just tried it on the back of my tsu9600, and it removed a LOT of the stickiness.  Multiple applications were necessary.  I could also scrape it off with my fingernail after the isopropyl loosened it up.  I got to the point that the surface was smooth and much, much less sticky, but there was still discoloration.

I then tried a Mr. Clean Power Eraser.  I got it wet and then squeezed most of the water out.  The white side really got in there and removed most of the discoloration.  It’s not 100% like-new perfect, but it is sooo much better than it was!

This approach took me about an hour.  If you’re lazy like me, and don’t like the idea of sanding and painting, this might be worth a try.  You can always sand and paint later if you don’t like the results.