Polishing or burnishing soft metal (brass, copper, aluminum, etc.) parts with Corn Cob Grit in either a barrel or vibratory tumbler is a popular process. Selecting the proper grit size for small parts or parts with complex geometry can be a real challenge.
Using a very fine grit will prevent the problem completely. Unfortunately, finer media sizes (i.e., lighter particles) can sometimes not provide the force/work necessary to achieve the desired surface finish. In addition, finer media can ‘pack’ into the interior areas of the part – like the inside diameter of the part pictured – making the media removal just as difficult.
There is no right answer to this problem. Balancing the required surface finish with the effort (time, labor, etc.) to achieve that finish will determine the appropriate process for each situation.