I have tried Iron-x, Bar keepers friend, muric acid.. ive also tried a lot of random weird things as well. The thing I stand behind the most is ZUD, its Hands down my favorite cleaner of all time. sometimes it takes a little more scrubbing but I have never lost gloss or hurt any signs. Ive even used it CAREFULLY on some painted sings with success. It cleans rust stains like its nothing. I wet the sign, and apply liberally and scrub just enough to make sure all the sign is covered then I let it sit for 3-5 minutes, finally I scrub the entire sign (spending more time in bad areas) with a short bristled brush and rinse. Love it!
The pictured sign is about 32"x32" and the whole process took 10-15 minutes.
