Yes emery cloth is the same as plumbers use, it comes in different grades (fine - course). Use the finest grade for a better finish.
GO SLOW.
Check fit often. Rather have to check and resand than to over sand and trash barrel.

Use shoeshine motion with barrel held in sturdy vise. Rotate barrel often to sand evenly.
No I have not done this on this type of barrel but it should not be hard to do.
Good Luck, and take your time.