Abrasive flow machining. Also known as abrasive flow deburring or extrude honing.
48-second video of extrude honing for an aerospace part:
17-second video demonstrating the process:
And I believe this is the original patent, granted to the Extrude Hone Corporation in 1974: US Patent #3,819,343. The basic formula for the abrasive listed in the patent is guar gum, boric acid, and borax.