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if it is the neck you need to clean i would tumble it. if it is the threads use a brass brush to do it.
blasting the tank is not a good idea as this is a more abrasive way to do it. like when they sand blast the paint off a car. not a good idea for the inside of a tank.
if you want to use a tank whip buy one from global. you can get two different length rods for doing 80's or pony bottles.
tumbling media consists of glass beads, aluminum oxide chips, and a tumbling expiditor for heavy rust.
Yes, that tool works well for most of the tank, but I use a different one for the neck. Actually, I like my other one better for most cases since it seems more durable. It has stainless steel wires that were inserted on an aluminum rod every inch or so. It looks like it was a semi homemade unit, but it works great and it was very reasonable price.
I also have one like the one in the link and another wire brush for the bottom.
This tool are much less cost that a tumblers and most certainly less expensive than a blaster.
I can justify owning all this tools since I have about 16 or so steel tanks that I use on a regular base. It is also a lot less mess and easier to use than a tumbler and I can do it dry and just blow out the minimal dust. It definitely takes care of any flash rust if any occurs after a hydro.
It would probably not handle any significant rust, but it would have to be fairly significant.