I used to have this Hydroflouric brightener they used on tanker trucks. I wouldn't recommend it to anyone, but it sure as heck did a nice job prepping a surface for polishing. I used to brush it on with a long-handled brush and rinse off with a hose from as far as possible away
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I got it at a truck stop, if you don't mind taking some *serious* precautions it might be worth a shot.