Created this matte painting using various techniques including the kit bash elements available in my Gumroad store:
Tech Heavy Pro Kit - https://gumroad.com/l/ztYeA
Junk Yard Kit - https://gum.co/XmTkp
I've made the .PSD for this artwork available for free at: https://gumroad.com/l/PjMZM
I started the process by arranging the 3d elements in 3ds Max and rendering them using V-Ray. The textures were created in Substance Painter. The metallic texture was more custom, while the rust texture was the default rust available in the program. Once i had the scene composed, I created multiple renders to give me as much flexibility and options as possible. I created a completely metallic render, and a complete rust render, and selectively blended between the two using various techniques. I also created a Vray edge render, to use as a mask between the two layers and many other render elements. After that, there was a lot of textures pasted over to add further grime, paint work, grading etc ..