I picked up a old but complete ShopSmith from the curbside a freebie find that has a varible speed drive then I added a Variac Autotrandformer to slow it down to 40 RPM's. The ShopSmith will handle 16 1/2" diameter objects.
With steel disk bolted to the faceplate that has 3 nuts welded around the edge allows for 3 allen bolts to bite inside the bottom skirt to center and drive the keg with a live center at the filler neck. After dressing down any sharp burs I went at it with a 9" body grinder / buffer with different grit flap wheels then buffing compounds.
All I had to do is hold the buffer on top and let the lathe do the rotating, no ass busting with the keg rolling around. This was for a friend as I do not care what the kegs look like under the insulating wrap as I want functioning brewing system not shining art work.