This is absolutely the best spray to free rusted steel bolts, nuts and studs. I work on marine equipment and vessels operating in salt water. Sometimes I have to remove a nut or bolt that is badly rusted. Sometimes so rusted the flat sides are mostly gone. I spray Kearney Rust Cutter, wait a minute, and the nut easily frees. Also works great on exhaust manifold nuts & bolts and for removing studs. Much better than the common solutions of light oil and solvents.
This is hard to find but really works. A friend gave me a can years ago and when I tried it it will bubble up and given a few minutes it will penetrate and usually loosen the rusted bolt. Try it for yourself.