I once watched a Japanese tamiya kit builder years ago and he used a technique to make scratch marks on the belly of the car to give it that look you get when carbon/metal runs against curbs etc on the track. Well I developed my own by pure accident. When sanding down and preparing if you forget to use the fine at the end it gives off the look when you paint over it that this has happened. If you notice carefully you can see the lines. I have an old weathering kit that I use to bring out the scratches at the end when its finished and it looks quite nice and not over the top. You can go as far as you want with models its just your preference really. I’m not a great one for it but sometimes a subtle used look is nice.
This is the weathering kit I use from tamiya. Item number (87098) I use the one for military models. Mainly because that's all the model shop had at the time but does work.
No where near finished but here is a close up. You will get the general gist from this. Once its dry i’ll put a bit of alclad silver and bronze through it and the weathering kit too.