I've never tried muratic acid, sounds like a good idea. Last sign that I removed rust stains from came out very nice, it's the neon union 76 sign I use as my avatar and is in forums/showcase. I cleaned best I could soap and water, little CLR which didn't do much for me other than remove caked on dirt. Then scrubbed with an ultra fine scuff pad and water/CLR where rust was. There like scotchbrite pads but very fine, had bought some for final sanding primer from auto paint store. I've only seen them at auto paint stores actually. Equivalent of like 1000 grit or finer I think. Lose a little shine but it comes right back by buffing with an aggressive cut cleaner, wool wheels, and electric buffer. Fast way to clean signs, I've generally haven't had much luck using chemicals but I'm not patient either.