Tyinksy.....Irn Bru would work much better than Coke:devil
Seriously though Geoff I use Phosphoric acid various mixure strengths on all my brass plaques and copper items and even cleaning old electrical connectors wires before re soldering.
If you look at any decent alloy wheel cleaner or rust/oxydisation cleaner you will see it is a high content usually of phosphoric acid.
We have a brand name in UK called Jenolite and you can just brush on and wash off. We also have another product called Cillit Bang but this is a very weak solution for the home use but does work and the dirty old two pence piece comes out the tank sparkly. I have also in the past used 'Silver Dip' another brand name here in UK.
Look for a decent brand spray bottle of alloy wheel cleaner in the autoshops and see it contains a solution of phosphoric in its contents. All others mentioned though will work too
I had thought of coke as Tyinsky mentioned - there are lots of youtube vids but I will go with a specialized cleaner with phosphoric acid. I have to call in at Autozone later so will check out the wheel cleaners and others.
I thought you might like this.....just my two pence worth and putting my money where my mouth is:devil
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.
If you have access to a parts tumbler, they can be very beneficial to the DIY'r especially with mechanical restorations. The small one from Harbor Freight for example, will suit most needs. Depending on the media used, they are wonderful for restoring the finish, removing corrosion and finishing metal stampings with production parts, hardware, tools, bolts etc. Go on Youtube to see the results. The plate is a great candidate for this.
You can sometimes find them used in the classified postings and they are not hard to build.