Doing it by hand with sandpaper is a nonstarter.
Also I don’t have a lathe :-/

  • AMillionMonkeys@lemmy.worldOP
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    2 years ago

    I’ve actually tried this! The problem is that the sandpaper is too thick to bend neatly in that tight a radius. Maybe I could buy raw grit and stick it in the cavity with spray adhesive…

    • Ugly Bob@lemmy.world
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      2 years ago

      You don’t want the whole hemispherical cup. Just a semi circle.

      The best and traditional tool for the job is a rasp. I recommend a Shinto rasp. Surprisingly fast and easy to use.