I use Bar Keepers Friend on mine. Makes a paste and run it in circular motions.
This is the way to go. Barkeepers friend or Bon Ami (Bon Ami is made for porcelain. It’s about $1.60 a can at Walmart).
Use a sponge (the kind with the scotchbrite side works better), a small amount of water, and some Bon Ami. It’ll make a paste, scrub sign.
That face plate would take less than 2 minutes and the rust stains would be gone.
I’ve done hundreds of porcelain signs with this stuff. Fast and easy.
Use on porcelain signs only*
This one took me all of 2 minutes.....