I just used nic a date to restore a date on 1920 Canadian 10 cents. I left it on there for about 30min. It did leave a nasty stain, but the date is clearly visible now. My theory is that since Canadian dimes were only 0.8 silver that year, higher copper content made restoring a date a bit easier.
http://goccf.com/t/375412#3219599 unfortunately, I don't have a before picture, since I didn't think it would work. But you can see from the obverse that it is worn slick. Only the 9 was barely visible before.