An O Gauge HUO hopper from Accurascale is the next challenge.
The first thing to do is to choose the colours that will be used for this, and another seven to follow. Each of them will be tackled as a separate task, in the hope that the completed set will all look similar, but definitely not the same as each other. The plan is to use Revell no. 84 as the basis for the rusty look, MIG pigments Dark Mud and Track Brown for shading in corners and around edges, Railmatch Sleeper Grime for the underframe and some other pigments for touching up at the end - probably Rubble Dust and Black Smoke from MIG.
Edited to add: When you reach Step 6 you'll find that I've been remiss in not mentioning that I started with a coat of Testor's Dullcote, so that the pigments would 'stick'. Thanks for pointing it out, Steve.
To check that the Revell no. 84 will not be too dark as a rust effect, I have airbrushed a thin layer onto some of the external bracing and hopper interior to see how it looks.