Just something I noticed the other day while cutting a lemon in half with a plain metal knife. This was not a stainless or plated knife. I left it on the counter for an hour or so after cutting the lemon, and I noticed that the acid in the lemon turned the knife a really dark blue like bluing on a gun. I washed the knife and it stayed that dark color. I wonder if that would work with a Q-tip dipped in lemon juice and applied to a spot that needed touching up on a gun???

Steve