I suspect you're right, but knowing myself I'd quickly get sick of sanding (6 faces with 5-6 different kinds of sandpaper), and I'd want an orbital sander. Then I'm pretty sure you want to polish the surfaces: lots of elbow grease or a polishing/buffing tool. If I had a workshop with space for tools and dust I'd probably go for it, but I'm doing this in the living room of a small apartment.
Here's the process as explained by a reputable epoxy vendor: https://www.youtube.com/watch?v=9-WYOK90KNo
Incredible work. I love it.
This would be a similar process to finishing any painted surface. Progressive through the grits, each time removing the scratches from the previous grit, wet sanding with the higher grits. You can then hand polish with polishing compounds you can get at any hardware/car accessory store. You can start with an orbital sander or a belt sander of some sorts. I've done this on guitars I've built.
If you're fairly close it shouldn't be a ton of work.
Tape or glue sandpaper to a piece of glass for a really flat surface. It works nicely for making consistent angles.
Source: holy shit can you get a knife sharp using this method.
Wow, never heard of this method, thank you! What kind of sandpaper do you use?
Any brand. It's the grit progression that's important. Watch Paul Sellers for tips: https://youtube.com/watch?v=bailuQUh2mY&pp=0gcJCfwAo7VqN5tD
I did a resin cast that had a similar "lip" at the edge (I turned a LGA CPU socket into a coaster), and I was able to sand it down fairly easily using regular sandpaper of progressively higher grit, producing a more or less perfect cuboid. The flatness was achieved by taping the sandpaper to a flat surface, and moving the part.
It's been a while, I can't remember if I used some kind of polishing compound at the end or whether the sandpaper alone was enough for a good finish.