Great thread. A while back my boil coil was dry fired. It got hot... to the point I thought I had opened a portal for Lucifer himself to ascend upon earth.
When I peered into the kettle I saw the coils glowing cherry red and orange. After they cooled the coils were purple. I scrubbed em, star san'd em. Purple.
Just purchased some barkeepers friend. Used as directed with soft cloth. Purple. Scrubbed harder and changed to blue scrub pad. They turned blue! And some brown gunk was forming on the pad. Traded the blue pad for green scotch brite. By God there is stainless steel underneath there. I tried to take this pic halfway through scrubbing. Top and bottom coils have received some treatment, middle coil yet to go.