I am also working on some 100 year old fireplaces in an old mansion that look like they haven't been touched ever. I am dealing basically with a soot/rust problem (see picture below). There is an iron/steel grate in the fireplace that we removed. there are many brown spots along the border of the fireplace box. We honed the hearth to 50 grit (marble) and treated the spots about 5 times for 5-10 minutes with rsr 2000, and then poulticed for a few days. The spots are a little lighter but still there. Anything more I can do other than continue poulticing? thanks,