Can the FRENCH GREY surface be on stainless steel actions?

Is it an applied treatment, like case colors? It sure looks

good engraved on a Mauser actioned 500 Jeffery cal. rifle.

In theory ALL stainless steels with maybe a rare exception WILL rust.

So in theory yes you could use any rusting finish on stainless.

It might just needer a longer time and maybe in a steam box to start to turn.

But as to if under the rust that forms you would have started to colour the stainless I don't know.

There are several ways to apply the French Grey appearance, and on stainless IMO the acid method would be perhaps the best. Brownell's will give you a free pamphlet that describes several approaches to the treatment and it's also described in one of their Gunsmith Kinks books.
Regards, Joe

I don't have my copy of Brownell's french grey method But lets just say I was not impressed with the simplicity of the finish.
In the description that brownells sends there are a few ways to achieve the same finish on different types of steel. One was a bead blasted nickel plated finish, then the other were varying degrees of acid etch. Aside from the nickel plating there is no surface coating or chemical finish being applied it is simply a mechanical surface finish that is no more corrosion resistant then the base metal itself.

When I was a Browning dealer, I was told by the folks in Morgan they used bead blasting in pigeon grades, etc.

Works slick! Nice and even (use fine glass beads) Finish with lacquer or similar

I use the metal prep made for the automotive industry. It's a liquid with phosphoric acid, so there is some protection to the metal.