Stainless

Not me that's stainless - it's the black, rear wing finisher I bought by mistake.

The photos above are both of them on the car, although they can't be stuck into place until after it has been painted in the final coat of Opalescent Golden Sand. 

Using liberal applications of paint stripper I removed the hard-as-nails black paint that was on the recently purchased finisher (I couldn't abrade it off else I'd destroy the shiny finish) and discovered that it is indeed made of stainless steel.

A couple of days in the open didn't produce any rust marks and I also did the magnet test. Mild steel is magnetic, whereas stainless steel, depending on the quality, isn't. Both mild and stainless steel are alloys that contain iron as a major component. Iron is naturally ferromagnetic; however, in stainless steel other compounds are added, such as nickel, which can render it less magnetic (or magnetically confused) by locking the crystals in an autenstitic structure. Adding 16-35% nickel suppresses magnetic phases.

In my last GT6 post I mentioned that the rear finishers are made from chromed steel, not stainless (much as I searched, I couldn't find any stainless ones). I thought I might be able to fashion some stainless rear finishers from a couple of stainless wing finishers, but they interlock into each other and so it's not possible with my limited metalworking skills.

I'll just have to make do with chrome.