[Farrell In Vancouver]

For a fairly good mechanical clean, I have strapped tanks to a cement mixer loaded with glass beads blasting media, small screws, and nuts and sealed off. Run it on the mixer for four hours and the flip the tank and run it again. ( Keep a count of the screws and nuts for later retrieval) You can alter the pitch of the mixer to get different areas. This removes a ton of scale and rust deposits and can be washed out with soap and water. I've added water to the mix as well to make a slurry that really cleans the surface well. Inspect it often and if not happy, tumble it for a few more hours.
Just my 2 cents, your result may vary.