Rust under the black
Don't get shook up about it, rust can keep rusting with out water and oxegen thats what the black converted acid does it converts it and seals it. if you sand it off you have opened the wound again let it be and drive and be happy. Do not try to cross bridges until you have to cross it. I have never had a problem with the stuff i use shelf life is only good for 7 or 8 months I buy a pint at a time. It says on the directions do not sand or grind after you apply the stuff. Its only a Car fellas.