So I got my hands on a free but slightly used turkey fryer and 30qt kettle from my in-laws. The kettle still had grease in it from Thanksgiving. I cleaned it out, scrubbed with a little detergent, rinsed thoroughly, then filled it with water and PBW and let it sit overnight. The grease is gone, but there are still a few spots at the bottom of the pot that I cannot get off. They are a dark red and black color, like something is scorched on. Is there anything I can do to get this off? I was thinking buffing or wire brushing it off or using something abrasive, but I don't know if this will destroy it for brewing. Soft scouring pads aren't working. I do realize the oxide layer is be gone. I plan on restoring it.