I used a couple of polishing wheels in a drill for mine. Bungings sell them with a coarse and soft wheel in two sizes and two different compounds (about $40 iirc).
If the kegs are beat up, use some fine wet and dry and water or crc first. Then use the coarse wheel with dark grey compound followed with the soft wheel and white polish.
That was my method anyhoo. Took me about 45 mins per keg with a few breaks here and there.