I thought you might like this.....just my two pence worth and putting my money where my mouth is
I got an old oxydised 2p coin and put it in my phosphoric acid. Mine is a commercial strong mix I got to remove rust stains embedded in my tiles around the swimming pool as the previous owner had cast iron furniture sitting on them.
I had to turn off the camera flash to get a picture as it came up so shiny. It does need a wash out afterwards with clean water but only in the jar for a minute.
I may just try the coke test though out of interest.
All the best