Roebic Sulphuric Drain Cleaner

This is the second time I have used Roebic Brand sulphuric acid drain cleaner. The first time in the bathtub, I didn't cover the drain, when I returned to the bathtub, it was covered with a rust color stain. It did cleanoff with time and considerable elbow grease. Now I have done it again! This time I covered the drain so the gases would cause the stain again. It didn't work, I still have the rusty stain in the tub. So what is the stain, and what is the easy was to clean it off. Mike
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