Stainless steel does not (in general) have- or need a finish. It is an alloy of iron carbon, nickel and chromium that resists rusting (NOT rust proof!)
Other finish can be applied over ordinary carbon steel to keep it from rusting. Bluing (a form of controlled rusting) and nickel plate- later satin type finishes do go on the ordinary carbon steel. .
Nickel metal compounds (and chromium) are highly toxic, nasty heavy metals- would urge you not to fool with them in the kitchen. Send the gun to someone that does that for a living, or look at some of the home coatings you bake on if you want a new look.