So, I figured I have the perfect tool for sculpting and just brute force removing material. It works nicely on aluminium cylinder heads, so there's no reason it won't work on soft epoxy. It's my die grinder. So in goes a solid carbide 12mm rounded die, and off I go.
So the interior painting process is one of applying a coat of undercoat, finding bits that are awful, going back to filler, cleaning that up with sandpaper and my die grinder, doing more undercoat, etc. At some point my OCD will finish and I'll apply the top coat.
Here's the first coat of undercoat. I know. it looks absolutely dreadful.