SANDING FOAM RUBBER! Holy smokes,...sure works great! How very cool!!
I used a 40 grit pad that I had on my 6" sander, but I think that an 80 grit sanding pad might work better - I think the material removal would be easier to regulate. And I found that a moderate rotational speed was best - too slow, and things 'grab', too fast, and you'll remove too much material too quickly.
But YES, you can sand foam rubber! Smooths it nicely,...
I'll touch-up a couple of spots (it's my first foam repair,...) and the old buns will be ready for re-installation.

Application of new upholstery will be covered when I can afford to do that. Hopefully in the next month, or so.
One yard of the dark red sierra vinyl that the upholstery will be made of is on it's way, so that I can inspect the material and cover the rear side-panels.
I was able to peel the old upholstery off of my original panels with just my fingers. They are washed, and I'll try to remove as much of the old contact cement as I can, before I install the new vinyl.
More soon.
Hope all of you have recovered from last night, and I hope you have a great 2020!
Happy Motoring!
Todd.