Work is always good. Any hardware store should have the acid; it's a common item for cleaning bricks and stone. You should be able to get product and usage info online for it. It will work, but rinsing it off might be an issue. You may have to do small sections at a time. I've found that good old Tilex will clean just about anything off of anything, and is always worth a try, although, while not acid, it does have a lot of bleach in it.
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