I've learned a lesson this winter about sanding epoxy: it takes 5 times more effort to get a smooth finish once the epoxy has cured for 6 months. I feel like I'm going through a piece of 80 grit every 1-2 feet! The other lesson here might be to make sure your epoxy coats are really, really smooth and drip-free when you build it up over the glass.
Just wanted to share...