For the start:- Where is the artstation link?- Why not post more angles?
Cool thread title to get the feedbacks! I gotta try that...
Look at dirty wheels, were does the dirt stay? how much of it is there? Does the colour change depending on how deep it is? Where does rust start? Does it change colour depending on where it is, maybe because of the sun or because of the material? look this things up and do it for every material.