bob haller wrote:

a buddy of mine would remove rusty tanks and put gravel or some such in
them and had a way to vibrate them. he would removel the gravel, clean the
tank and solder any holes

A length of chain works pretty good and is a lot easier to fish out than
gravel. Often the filler has a neck that extends a little way into the tank
so gravel won't just pour out.

https://www.youtube.com/watch?v=GNEwIxWUJjU

I'm NOT recommending this technique but it certainly is creative. It's a
little too close to the log splitters that are a big screw that bolts onto
the rear hub for my liking.

All in all, I'm happy with the phosphoric acid approach rather than
mechanical stuff. Phosphoric is pretty benign; read carefully and you'll
find it's in your Coca Cola which is probably why Coke is used as a rust
remover sometimes.