Wife has said I'm sick for decades but thanks Skip.
Henry, it's a big subject but glad to share. I welcome dialog about building Pochers.
It's no secret, no magic and no commercial products. All the aging materials are common items and not powders or mixtures from commercial companies. I always felt the pigment powder sets, although easy, were too fragile and not permanent enough to use. Besides, a large assembly like this engine must be handled many times during the process and that would destroy those fragile finishes.
Where to begin? Well 1:1 automobile experience is important. Having a background in fine art helps too. Art college when I was a kid, dabbled all my life in it and owning framing shops contributes to the knowledge base.
I spent time restoring vintage and exotic cars in shops. There you see it all-decades of corrosion and abuse as you strip them down for restoration. Some of the most abused parts, you learn, are the various fasteners-they chip and rust their finishes in various ways. Building and maintaining my 1:1 race car for near 3 decades teaches you about various materials and finishes. So the simple answer is to go out and view 1:1 cars of all types (body shops are a great source) and photograph the age and corrosion bits. And remember, it's not just rust. Before rust comes corrosion and different metals and fabrics corrode in different visible ways. A car will have different degrees of corrosion leading to different levels of rust. Heat is a major cause of corrosion so learn the hot areas and see how they corrode naturally.
You cannot achieve this level of detail unless you work in 1/16, 1/18, 1/12 and 1/8 scales. Much more easy and realistic. In smaller scales, the aging must be so subtle it's nearly invisible. But it can be done to a degree if you acquire the touch.
Here are the numerous materials I use on the Rolls engine and chassis:
acrylic craft paint / Future / Windex / Rub 'n Buff / Dullcote / brushed gloss enamel clear / brushed Tamiya Smoke / make-up sponges / many types of art brushes / Q-Tips / scuff pads / powdered graphite / auto compound / airbrushed Alclad(s) / Testors Metalizers / Bondo and microballons in CA as fillers.
Here's a brief list of what does what:
craft paint; a good flat finish for general corrosion and age streaks. Thin washes (add Windex) are best, layered when they dry. Mix colors to get what you want; I start with light and dark browns, black and ivory / off whites, all mixed to my taste. Don't forget mixing grays to subtly change a black surface finish.
Future: makes a good barrier for layered finishes.
Rub 'n Buff; aluminum streaks, golds for heat and mild corrosion, metallic scuffs and highlights. Use like a woman puts on make-up. She highlights her cheeks, lips and eyelashes. Tip; thinning with enamel reducer or turpentine and dabbing on with Q-Tip gives best results-apply it thin and add more if you need.
Dullcote; ages a heat area by altering the finish of a part but not its color. A shiny steel or aluminum part turns dull from heat or road dust.
enamel clear; makes drips wet and heavy; apply with restraint.
Tammy Smoke; subtly brushed on adds dingy look to chassis parts-it's very thin and subtle.
make-up sponges to daub various acrylics. A sort of dry-brush technique (less is better). PRACTICE on paper or towels.
powdered graphite; rubbed into a surface puts on a metallic sheen which catches light but is not a 'gloss' type finish. Use on discolored metals (from heat or sun fade). Makes metals more 'real'.
Alclads and Metalizers should be applied by airbrush and can be layered for colors different than the bottle color. They can be buffed and polished too.
auto compound; very fine compounds can be used to polish bare plastic for a different finish (the Rolls valve cover).
The various brushes, pads etc are all useful tools.
Techniques: remember corrosion starts as 'age'. A simple discoloration due to heat or sun fading is first. Then a gradual lifting of the paint or coating shows as a dulling and light color (grays or the family of off-whites to tan) which eventually breaks into rust areas. Those range from light surface rust to deeper heavy rust. And it's not just 'rust' that shows age or condition. Parts removed frequently by mechanics have chipped edges and scuff marks-and sometimes greasy hand marks. Wires age and get brittle and fray-they discolor from heat and age. See the generator and plug wires on the Rolls. Again look at real parts (valve covers, alternators, carburetors, brakes, shocks, driveline) for guidance.
So just don't put a blob of rust somewhere; put a subtle wash or dab of light color down first then add rust (lighter at edges, darker at center) within that. The edges will be lighter and the rust more gradual and natural. An area can have 3 or 4 colors within it which make it up. Also remember rust is most common where joints collect moisture (when parts cool), not just big roof surfaces on small models. Look at all the joints, brackets and low spots in the exhaust system and you'll see it.
Review your work often. View your project as a 'work-in-progress', meaning don't just glue part A to part B and color it. Plan ahead the effect you want and only fasten parts permanently when you don't need access to their backs or sides anymore. Then, as you add parts, you may go back to soften an effect you overdid or emphasize it more as you view the whole assembly. This has been a great deal of what I'm doing every day to get this final effect.
Sorry for the lecture Henry but that's my method. Hope you experiment and begin to use these techniques with your already excellent skills. Maybe consider building a Pocher classic???
Edit: forgot to mention Tamiya clear blue, yellow and red. I use these for parts of the exhaust to show heat bluing and discoloration before rust starts in the muffler heat shield on the chassis (see earlier pictures in thread).
In 1967, Cadillac offered an option called 'Fire Mist'. I painted my '67SS with Cadillac Fire Mist Blue. Was the finest flake (I think it was early mica) I ever saw.