I have never had it leave a residue other than a slight film where the rust has converted. You will want to immediately wash the part in water after you take it out of the bath and then dry it with compressed air. Wire brushing the outside as you clean it would be helpful.
You might want to try it first on something else if you are concerned about it.
The critical thing is to get the polarity right, because if you get it backwards, it will remove material.
Be sure to wear heavy duty rubber gloves if you are using lye. That slippery feeling if you get any of the lye solution on your fingers is the skin cells sliding off.
Be sure to read FIT's article.
Kurt
