"车厢面漆" can be translated as "carriage exterior paint" or "coach exterior finish" in English, depending on the context.
"Carriage exterior paint" emphasizes the paint applied to the external surface of a railway carriage or similar vehicle.
"Coach exterior finish" is a broader term that can refer to the final coating or treatment applied to the outside of a coach (whether it's a railway coach, bus, or other similar vehicle), which may include not just paint but also other protective or decorative finishes.
In many cases, especially when referring specifically to the paint layer, "carriage exterior paint" would be a more precise and commonly used term.