How to add some dust on this material?

I am using a material which I got from cyclesmaterialvault , to be exactly metal 7. So it has some scratches etc. I want to add some more dirt on it as it still looks somewhat in good condition (my personal oppinion)

I am not sure on how to do this as the material node is pretty large and I am not that experienced with shading yet. A bit worried that I might plug something wrong leading to it breaking down on me.

https://i.stack.imgur.com/QUByO.jpg

Check hereā€¦