Hi Jenn,
it's always best to remove excess before the resin is fully cured, usually the day after mixing. The epoxy will be hard enough for sanding or scraping but not such a tough bastard like you're experiencing it now…
I don't know what tools you have in your workshop but my first choice would be a Fein Multimaster (oscillating tool) with a scraper or saw blade attachment. Chisels or handplanes also work well most of the time. If you want to use a sander, make sure you're using a hard pad (a soft pad will dive into the wood but leave the epoxy in place). Also a small block of wood or cork with some 80grit sandpaper should do the trick.
if you make a mess you can always mix some filler and no one will ever know you screwed up…
Best,
Matt