What's the rough tape used on 1/8 fuel tank caps?
 

I've tried searching this site and google but no answers.

On the pictures of many 1/8 cars, I see a red tape that has a sandpaper like quality to it that 1/8 racers use on the flip cap of the fuel tank. Collari and Pietsch use it

Does anyone know what it is or how to make it?

Sandpaper ....... cut to shape ..... shoegoo it ..... for grip when opening tank lid ..... if not one slip of the finger + 0.5 secs lost ..... loses world championship .... but for us mortals ..... no need to bother :p

Or Grip Tape.

http://www.skatewarehouse.com/griptape.html

^ This is what I always used...