A *large* thick toothed file will work wonders, and be it's own reference as to what "flat" means.
That said, "home" sandcasting doesn't strike me as particularly easy.
I had them made at a casting shop, which already had the melting pots, ovens, "sand boxes", sandblasting equipment, grinders, etc.
As to why I made a lot of things, Argentina was a Tariff protected market until 1976, where we "opened" our Economy, under bayonet pressure, paying 30000 "dissapeared" to make it possible.
We also went from 4% to 35% (real) unemployment and increased our External Debt to 50x the original one.
In 1965 *we* sold electronics to Korea, made everything (including Mallory capacitors, CTS pots, Jensen speakers, FAPESA tubes, Texas semiconductors) except color picture tubes.
*Anything* which had a market could be made locally at a profit.
There was a short "Spring" in the 80's where we even made submarines, Saddam's missiles, AMX tanks, sold nuclear reactors to Canada and Australia, etc.
Then our Industry got clubbed to death in the 90's, until now.
Oh well.