My shop sprays routinely silver paint over the test stamp after hydro. A few dives after it's first hydro, it started to show tiny amounts of rust in the stamping, so I cleaned it up a little, masked around it, and sprayed a couple of coats of cold gal, followed by a couple of bright silvagal topcoat. I now do this after every hydro. I also spray white over the old one. Looks nice, no rust, and makes it easy to find the current test stamp... I have plenty of scuff marks on the cylinders, but none than run rusty.