the epoxy about 4 weeks oldNormally, 80# and fresh (less than 24 hours) epoxy doesn't feather well. Wait another day (48 hours) sands much better.
It definitely doesn’t seem like it would be durableI would be concerned after 4 weeks. 80# doesn't feather like 180# would, but shouldn't scrape off with a fingernail or even a razor blade.
80 grit da, no acid treatmentLooks like it's not feathering at the metal to initial epoxy coat? What was your prep like on the bare metal? Did you use an acid treatment?