Should clean up a grade or two depending how much of the porcelain finish remains. Softscrub would likely clean up the white porcelain finish but try a small area first. Simplegreen is another product that works well with a porcelain finish. X3 cutting polish is great for cleaning up porcelain if you have a buffer handy and can be found at
www.systemoneaps.com expensive but worth it! Feed-n-wax is another option after cleaning the sign. Zud cleaner would be too abrasive for the porcelain and could damage the finish.
