If you actually look at most modern gondolas out there today, you’ll find that most open top cars from the last 30 years or so actually don’t have any interior paint or coating, but instead the interior of the cars are raw, unpainted steel (other than perhaps some overspray from the painting of the exterior of the car).
Of course, with some exceptions, most model cars do not reflect this and instead of masking and painting the interior a rust or brown colour (which would really still need to be painted/weathered for a proper rusty steel look anyway) the whole car, interior and exterior, is painted the body colour.
I was having a bit of a painting and weathering weekend this weekend, and got out this sextet of Atlas Trainman SIECO gondolas to paint the interiors “rust”.
To get a good steel colour, I start by giving the inside of the cars a good base coat of Gun Metal. This gives a nice dark metallic colour as a base to work on. Then the rust colouring is added with several very thin coats of Rail Brown, Roof Brown and Rust. These brown/rust coats should be very light and thin, not going for solid coverage and allowing the lower colours to show through. The layers of colours and natural variation created by the light, uneven coats of brown/rust provide a nice rusty steel look.
I may complete the weathering with additional application of rust-coloured powders and some bits of debris from former loads, but the basic painting of the interior to change it from the main body colour (bright blue for the two GTW cars) to something that looks like unpainted steel really gets the cars popping with minimal effort.