Now then Kev,
There are no 'quality modellers' on this project, just a man in a shed having a bit of fun.
About this distressing, wear and tear, that lived in look or whatever phrase suits you. It seems sensible to me to duplicate what happens in reality rather than applying 'wear' on top of a finish. By undercoating with a metallic finish, in this case 'steel' then applying the service colours over that one can then abrade through the colour coat back to the 'steel' to replicate wear such as scuffing etc. That's the theory, we'll see how it turns out I reality.