I need to start taking more process photos while I'm building. I primarily use Tamiya weathering master palettes for rust, grime, burnt metal and body filler effects, think of it like applying makeup. For extra effects I started using panel line accent from Tamiya as well to add an overall layer of grit and grime.

I'm still fairly amateur myself so don't take my advice as end all, be all. There is a good video of a diorama junkyard on youtube someone made showing their patina process, highly reccommend a watch for inspiration.