I have always used a wire-wheel drill attachment to clean rusted tools and parts. It works like a charm, and rarely mars the surface. Then I put a spot of WD40 on a rag and wipe it down. Good as new! Almost... hehe!
This works great. I've done it several times with good success. Google it and you'll find detailed instructions. It may seem difficult but it really is very simple to do.If you have a number of tools to clean you could try electrolysis. It is a fairly simple process using Boraxo in water and running a low voltage current through the liquid. The tool is attached to one side of the circuit, and a piece of iron is attached to the other side of the circuit. Both are immersed in the water, with space between them, and the current carries the rust away from the tool and deposits it on the piece of iron. I believe the tool is attached to the negative side of the circuit. A car battery hooked up to a charger is a good source for your low voltage.
A battery hooked into the circuit should allow any charger to work. It doesn't have to be a really good battery.Just a caveat regarding electrolytic de-rusting. It is line of sight. That means if you have only a single anode on one side of your tank, only the side of the cathode (that is the thing you want the rust removed from) that directly faces the anode will be de-rusted. The way I do it is to have several anodes connected together that surround the piece I want de-rusted. Just MAKE SURE the anode(s) and cathode(s) do not touch.
I use a battery charger for mine. Not all battery chargers will work because some will detect the "short circuit" and shut off. Mine does that in "charge" mode, but if I switch it to "start" mode it works fine.