I also use a Spindler, but one gotcha: beware of denser materials ripping the center pin out. I can't recommend for cork <frown>. However a finishing nail ground-down to a triangular point has been working well for me for cork (IIRC there are directions on doing this in another thread.)
I found that Craft Foam Marshmallows sand down just fine. They're softer than other solutions but $10 for a sack was hard to pass up. They're soft like Flymen Surface Seducers but paint more like Rainy's Bass Pops. One pass with the coarse grit then one pass with the fine grit and they're as good as anything I've bought.