Citric acid is a relatively quick and inexpensive way to remove rust. The only challenge you may have is finding it. We get ours at All Seasons in Nashville. It's an organic gardening store that also sells home brewing supplies. The acid comes in powder form and runs about $3.99/lb.
All you need is a container that is sufficient to hold the item you want to clean and deep enough to submerge it in water. Mix 1 cup of citric acid per gallon of warm water and pour over the item. The length of time required depends on the size and condition of the tool.
The two pieces shown took about 2 hours. They were pretty rusty, as evidenced by the head of the tin snips. Click here to see an after pic of the tin snips; here for the wrench (notice the paint that was revealed).
As with electrolysis, when tiny bubbles form on and rise from the tool, the process is working. When the bubbles stop, the process is complete. After removing the tool from the water, simply wipe it down with a nonmetallic finishing pad (e.g., Scotch-Brite) to prevent scratching the metal.
Another great thing about citric acid is it's nontoxic. In fact, it's edible! You can even reuse the mixture on another project. Simply strain out the impurities and store in a plastic container. You'll know the water is too dirty for reuse if it looks like this. If you prefer, you can simply pour it down the drain. It's environmentally friendly! To see how these drill bits turned out, click here.
There are a couple of drawbacks to reusing solution.
First, if you've previously cleaned an oily item with it, the oil will
remain behind. With that said, even if you clean an item in clean solution, the oil will resettle on the tool. For that reason, the tool appears black after it's removed from the solution. Of course, it's nothing a good wipe-down won't take care of.
Second, the directions on the citric acid we purchased states to use warm water - something you can't do with old solution. Additionally, we've noticed that warm solution is more reactive. Either way, it's an inexpensive product. When in doubt, make a trip to the store.
Timing is critical
It's important to note that most items in a citric acid bath will continue to "bubble" as long as they're submerged. However, just because the metal appears to still be reacting doesn't mean it should stay in. It may be that a piece of rust that has broken free (or is in the process of breaking free) is still breaking down, causing the bubbles. Remember that you're going to wipe the tool down when you take it out of the bath. You'll likely be able to take care of any remaining small bits then.
As an experiment, we left a steel tool in a citric acid bath for two weeks. The result was frightening. Citric or not, acid is acid, and the outcome was what you might expect. The lettering on the tool softened during the process, with some of it actually disintegrating. Similarly, threads softened and wore away.
While most items will require 1-2 hours, the amount of time you leave an item in will vary depending on its size and severity of rust. We recommend never leaving an item submerged for more than 12 hours.
Citric acid vs. electrolysis
Citric acid and electrolysis are both effective ways to remove rust. In most cases, you'll be able to decide which method to use based on what you're cleaning. Citric acid is great for cleaning small items or items with multiple parts. That's because electrolysis requires that each item you're cleaning be part of a closed circuit. Further, when we restore a tool, we try to do it to the fullest extent possible. That means completely disassembling it. Imagine having to attach an annode to each tiny screw from a level or hand plane. It's a lot of work, and it's really not necessary when a citric bath will do the same job with a lot less work.