View attachment 206188
https://www.harborfreight.com/520-lb-rust-cutting-resin-abrasive-tumbler-media-63672.html
I've posted this before...
Using this media, I tumble the tank in my clothes dryer. The media is pure resin, no abrasives, and is extremely hard. It's hell on rust, but does not harm good metal. It used to be a Harbor Freight-only thing, but is now sold lots of places.
Clothes dryers tumble too fast -- centrifugal force keeps the media in one place, it does not tumble through the tank, but it still moves enough to work. You just have to keep repositioning the tank every 5 minutes or so. It really works great on all of the outermost tank walls. Getting it to work on the tunnel is a bit of trick, but with some ingenuity you can get the media to clean it too.
You want to really stuff the dryer with pillows, cushions, sleeping bags, etc. I have a big dryer. Tanks may not fit in all consumer dryers.
I just did a tank about a month ago. It really does a great job. Couple things I learned on this latest job: Use petcock blank-offs, as has been said, but also make sure you have a tight gas cap, or make a blank-off for it too. Otherwise you will get red iron oxide (rust) all through the dryer -- that stuff is incredibly fine. And, if you do this to a tank with good paint, wrap it in several layers of stretch wrap, like is used to wrap palletized loads. I suppose Saran Wrap could be used -- use the whole package.