The plastic models of stainless steel cars typically have silver paint that lacks the mirror sheen of real stainless. Even the plated finishes of the brass models don’t have a realistic stainless look, and plated cars from different makers don’t match.
I hadn’t found a finish I really liked, though, until I saw Keith Kohlmann’s Model Railroader article about painting with Alclad II Chrome lacquer.
First paint the car with a gloss black enamel undercoat. This acts as a background for the mirror-like finish of the Alclad II.
Let the black dry thoroughly, then airbrush the car with the Alclad II Chrome. This lacquer is thin enough to spray without thinning. Be careful not to touch the chrome finish until it’s sealed.
Seal the Alclad II with an overspray of Testors Metalizer Sealer or similar clear lacquer gloss coat. Then you can handle the model, apply decals as usual, then seal them with another clear coat.
From The Model Railroader’s Guide to Passenger Equipment & Operation (Kalmbach Books, 2006).