My writeup for the same thing...
You need some sort of plastic container that the items you want to clean will fit in, an older battery charger (the newer automatic ones will think it's a short and cut out), a piece of steel (NOT stainless steel) that you can sacrifice, and something to clean. You'll also need either washing soda or TSP (Tri-sodium phosphate) to mix into the water to make it conduct electricity and clean the metal. I've heard TSP works a little better...I'm probably going to try it in my larger tank. The first one has washing soda and it works well, but parts will flash rust when removed if you don't scrape/wire wheel them immediately afterwards.
I use old mower deck blades as the sacrificial electrodes. Take an angle grinder and grind the rust off of the sacrificial metal so that bare metal is exposed. Rig up a piece of wood or something to hold it in the water, with a small portion above the water so you have something to connect the battery charger to. You don't want the alligator clamps of the charger to touch the water. Clean up a little spot on the part you want to clean that'll stick out above the water so you have something to connect the battery charger to. Next, fill the container you want to use with whatever amount of water you need, and then mix in washing soda or TSP until it'll no longer dissolve. For TSP, use 1/8 lb. per gallon. Shoot for something close to that. Put both pieces in the container with the water mixture, making sure that enough is sticking out so you can connect the battery charger. Also make sure they aren't touching underwater (or anywhere, for that matter).
Now, connect the battery charger up before you plug it in. The polarity is very important and is as follows: POSITIVE clamp goes on the sacrificial metal, and the NEGATIVE clamp goes on the part to be cleaned. After it's connected, plug it in. The battery charger should show a current of 1-3 amps (or more if you have a larger battery charger). You'll see bubbles coming out of the water near both pieces of metal. Make note of where the current starts...if it drops, you'll have to clean the sacrificial electrode. Make sure you do this in a ventilated area with no flames or sparks, as hydrogen is what's bubbling off of one of the pieces of metal.
After a few hours, you can remove the part from the mixture (unplug the charger first) and scrape off the paint and black areas with a razor blade and then wire wheel. The rusty areas turn black from the carbon that was in the steel that rusted away, and you need to remove it before painting, which is the reason for the razor blade and wire wheeling.
It's the easiest way I have found to clean parts.
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