Here is how I painted my Kill Team Arena metal doors. This method is based strongly on Next Level Paintings "5 Easy Ways- Weathering Metallic Surfaces" video. I would recommend checking it out, there is lots of great advice there.

Metal

1. Basecoat with any old silver. I used Leadbelcher as a primer and a base.
2. Spray an orange-yellow roughly, over any areas where water and grime may settle. I used Bronze Flesh Tone and kept it pretty thin.
3. Being slightly more precise, Use a right brown to cover a majority of the same area again. This time be a little more precise. I used Beasty Brown for this step.
4. Trace rivets, panel lines and corners from the previous step as closely as possible. This time use a red brown, like German Red Brown .
5. Spray the whole surface with a mix of washes, to get a grimy colour. I used equal parts Nuln Oil , Seraphim Sepia , Athonian Camoshade and Strong Tone . I gave it a couple.of passes and adjusted the ratio to get the shade I wanted.
6. Drybrush with a light metal, just enough to pick out edges and rivets. I used Necron Compound .
7. Take a little Ryza Rust and add some water. Pop the watery mix around rivets and recesses, try and concentrate it on the darkest red-brown areas.
8. If any areas or rivets look to plain, add some visual interest with streaks or additional staining. I took some Skeleton Horde and went over rivets and recesses that I hadn't given any attention too. You could even use some super thin Skrag Brown if you wanted a lighter rust effect.

Lights

1. Paint the the whole light white. I would recommend a white with good coverage, like Corax White . Dont worry about being too neat.
2. Spray a heavily thinned pure white over the whole light and allow some to spill a little onto the surrounding area. Make sure this paint is super thin. I used a white ink, but Dead White is good too.
3. Spray the colour of your choice (I used Aethermatic Blue ) over the area covered in the last two steps, spilling out a little further again. Be sure to only hit areas that light would reach. I would recommend painting in short, light blasts.
4. Re-establish the white selectively, in the places closest to the light source. I used my old friend Dead White .