Anything acidic will get it off. You may find muriatic acid (hydrochloric acid) sold as Brick Acid? or if you can find a low cost descaler it will probably be phosphoric acid based. Oxalic acid will work but might not be that quick/might need a lot.
Whatever you use, think about protecting your skin, clothes and eyes, and how you will get rid of the waste - and make sure you thoroughly rinse it all off.
Would any of the household limescale removers work? I was thinking of the stuff you see advertised on TV???
Probably not strong enough, but if you have some at home, It could be worth spraying/brushing some on, or mixing it with walpaper paste to make something to paint on and leave for a while.
Probably will not work, but might be worth a punt before going down the hydrochloric acid route?
I've used Cillit Bang Limescale & Shine for small areas of worm casing on tubes and around engine mounts. You have to leave it on for quite a while and then rub off with a scotchpad. It works well though. It has phosphoric acid in it.
Either way, I'd recommend keeping the acid dilutions weak and allow them to work over an extended period of time.
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I'm sorry, but there IS no Mars Bar.
Nice Job Alex...Got any Finger nails left?..
When you see how well made the Old Avons were put together (and Designed).. it makes you wonder where/how they went so wrong! Yet Another case of British short-termism and managerial ineptitude no doubt! .... (Posted Bits this morning)
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A clever Man learns by his mistakes..
A Wise Man learns by other people's!
The Road to HELL ..is Paved with "Good inventions!"
but I don't want to mail order something if I can get it locally 
.... (Posted Bits this morning)
