Thanks for the comments!
@Urauloth I will be making a full guide on this scheme but for now I can tell you that for the rust I used Typhus Corrosion (for texture) and Dirty Down Rust – which is a somewhat unpredictable product, but unparalleled in terms of realism. I just need a bit more practice with it.
Moving on, I've returned to Astartes (literally!) for this Retributors space marine...