I'd want to see the end result. To start the effort, I too suggest a thorough search of real locos and a read of online resources.
it is a start
A pessimist sees a dark tunnel
An optimist sees the light at the end of the tunnel
A realist sees a frieght train
An engineer sees three idiots standing on the tracks stairing blankly in space
I'm not sure that a consensus is achievable here for various reasons.
Anyway, here is what I do for my diesels:
1) I cover the windows and lights with masking tape and apply a light coat of dullcoat.
2) I brush a coat of dilluted black acrylic paint over every part of the loco, except for the windows and lights. Mix is about 10% black, 90% distilled water, with a few drops of alcohol to make is stick. Experiment the mix on an old car to make sure you are comfortable with the mix. You should hardly see the black when you cover it - multiple coats are desirable. I use a fan to accelerate the drying process between coats.
3) Trucks: I paint a first coat of grimy black or graphite. After that has dried, I lightly drybrush the trucks with a light brown color. A little goes a long way... An airbrush works great for this step if you are comfortable doing that.
The above will remove the toy train look, but still keep it fairly clean. I'm not a fan of over-weathering, but that's a personal preference, obviously.
Simon
OK I'll dive in. Back when model trail events were live and in person I gave a two part clinic, part one being "weathering patterns on rolling stock" (prototype photos) and the second part being how to capture those looks without using an airbrush (live demos or examples to be passed around -- so the crowd could see what I was doing on stage the demos used junker Lionel car bodies). I wonder if I'll ever get the chance to give those clinics again.
The goal of my clinic (which was created to give at a prototype modelers meet) was to contrast with what one more often sees at a prototype modelers meet where the modelers pick a particular car or locomotive, gather photos of every side taken at about the same time, and try to replicate every little scratch, rust spot, worn off paint, blemish, ding and dent. The thing is, they model after what they have the best photos of. Often the photos come first and that controls what they elect to model. But what if you want to model something but don't have photos, or have photos of only one side for example? Many of the prototype modeler types would not go there -- that is not what interests them. They won't guess. But for the rest of us, that is where the idea of weathering patterns comes into play.
Anyway, my point being it does pay to study prototype photos for ideas like this, even if there is no one prototype you are following (or that even exists). So what does one learn? Here are just a few obvious examples
OK let's focus on 4. Paint fades - and for example that ex Conrail unit would be unlikely to have so intense a shade of Conrail blue yet have the lettering and logo be so faded and worn away. One way - risky - to fade paint is a light spray appliation of diluted isopropyl alcohol. I do this but results are unpreditable. I also add a little india ink to go into cracks and crevices and highlight detail. Another way is pastels or chalks but NOT using white which is often chosen but rarely convincing but use rather a lighter shade of the same color. Michaels crafts sells a very nice selection of pastels in gradations of color for a low price - find the closest match to your color then back off a shade or two. I am aware that chalks and pastels often disappear when sealed with DullCote. Following the advice of a modeler named John Ferraca, I apply DullCote first so that there is a toothy surface for the chalks or pastels to adhere to. This might be a repetitive process. I do suggest a final DullCote to avoid smuding with fingers. Someitmes you can get away with no sealing coat on structures but not on rolling stock
As for the ex "Kodachrome" unit, I remember those well and the paint did fade but more obviously got very grimy quickly, whether it was SP or SF, even when the merger was still up in the air. From tunnels perhaps. Again one way to add grime is to use black or gray chalks/pastels, but another way is to use a darker shade of the basic color.
I use make-up sponges for applying and spreading chalks and pastels. The ones I use look like little wedges of white sponge that you tear away from a sheet. Pretty cheap at Walgreens. And here is a personal safety tip: buy your own. Don't take your wife's.
I mentioned adding a bit of india ink to an isopropyl alcohol spray so that details would get highlighted. I believe it was Montford Switzer who showed in an article how to use a VERY sharp #2 pencil to highlight details such as the doors on the side of a diesel, cast on handrails, etc.
Those are just a few ideas but I'd add another message from my weathering clinic: weathering is a skill. Skill comes from practice. Then you tackle the actual project. Swap meet junkers, even if not in your scale -- in fact might be more useful in a bigger scale such as Lionel -- are your friend when it comes to practice.
Dave Nelson
Dawn gave me some of her old make-up powders. They are various shades of brown. I use them to do any 'weathering'.
David
To the world you are someone. To someone you are the world
I cannot afford the luxury of a negative thought
The idea was to make it a collective group project to see what appears.
Shane
First thing I would do is look for prototype photos, and plan from there.
Mike.
My You Tube
Thought this would be a fun idea. I have two units I will be weathering. I can easily do that. Thought it would be more fun to have others suggest what type of weathering and where it should appear. Once a concensus has been reached I will do it next weekend. see how it comes out.
First This unit is owned by the prodecessor of the NVSRR. and is so marked. The NVSRR does keep the quipment relatively clean. With that, paint does age. It runs in central PA the history of the region is Iron ore, pig iron and limestone. Limestone still being mined because it is exceptional quality.
nextLease unit. Yes I thought of some conrust on the nose at least. How shall we do this one?