I have had my share of disasters learning to do weathering. Best advice I can give is go slowly and don't overdo it. It is much easier to add more but no good way to remove what is already there. Remember you are adding paint to an already painted model. If you have some old models which you are going to strip and repaint, try out the weathering on them. One thing will become apparent, it doesn't take much paint to weather a model, easy does it!
When a unit is placed on the road several factors cause the painted surfaces to change in appearance, this is referred to as "Weathering".
Weathering happens to painted surfaces when exposed to sunshine, rain, snow, sleet, dust, and spills which cling to or attack the finish. In the 50's and 60's many railroads used wash racks and phosphate detergents, this caused the painted surfaces to fade quite rapidly. Items which leak grease, oils, fuels etc also cause the finish to "weather". The track itself adds a small contribution due to its rusting. When the train is running in the rain and as the track dries the rusty water and dust is deposited upon the running gear. This is why many engines and rolling stock seem to have a dusty, rusty look to the undercarriages.
How can we simulate weathering? Several ways, as our models have painted surfaces, we can change the look of this paint. A common method is to spray a fine mist of "Grime" onto the body of the model. This is the first step in doing a weathering job.
Here is a use for those half used bottles of paint! We will use them to make some colors to resemble Grime, Rust and combinations of these.
Basic "Grime" looks like this.
"Grime" can be made using white and a small amount of black. Proportions are difficult, but in general use 1/2 ounce of white to one or two brushfuls(#3 size)of black. You want to wind up with a light gray , then add 1/2 ounce of thinner. To spray always use a strainer in the suction line. Adjust the airbrush to give a very small spray pattern. Always try out the spray patern on something other than the model. I like to spray upwards on the model, using a piece of manila file folder to mask the underside of the trucks. A light application along the walkway edges or in a curving pattern on the lower nose of a cab unit is all that is required. Lightly spray the top of the model, basically in the center. A trick used by some is to apply a piece of masking tape to the windshield in the shape the wipers make before spraying the "Grime" When the tape is removed looks as if the windshield wipers have been working.
Basic rust looks like this
"Rust" is made using black and red, other dark colors can be mixed in as well. Use more red than black try 1/2 ounce of red and 1/2 capful of black. Application of rust can be easily overdone. I get the best results by applying the rust color with a brush. Study pictures of the originals to see where rust appears. There is always rust around the battery boxes and edges of walkway, on the step treads, on walkways where the doors open and crews step out and on the truck sideframes. I like to use a flat 1/4" Polly S camel hair brush, dip in the rust, rub on a cloth to remove most of the paint and draw the brush downward from where the rust is beginning. This technique is referred to as "dry brushing"... I also make use of "Instant Age" and "Realistic Rust" made by Smoky Mountain Model Building and Supply, E-Mail them. It comes in a 16 ounce bottle for $7.95 each and should do a thousand models. The "Instant age" looks like "Ogrime" and is great for extreme weathering, but easily overdone. The "Realistic rust" looks like "Orust" is great for weathering truck sideframes on rolling stock, also easily overdone. Both are made for brushing on and allowing to run down from the top or settle in the cavities.
Combining grime and rust gives "grust'.
"Grust" is used the same way as "Grime". This can simulate mud splashed on the running gear and walkway sides and the nose of cab units. Railroads that operate in the areas where reddish clay is prevalent often exhibit this characteristic type weathering.
Combining oil and grime gives "ogrime".
"Ogrime" is made by adding more black to the "Grime". Just make it as dark as you wish. Remember to add a bit more thinner to keep it transparent. Apply to the roof tops around the fans and exhaust stacks with your airbrush. A light application on the top is all that is required. Apply to the intake and exhaust grilles on the sides with a brush. A 1/4" flat edge brush works better for painting in the intakes and exhaust grilles. The thinned "Ogrime" will settle in the grilles and allow some of the original finish to show. Use straight black to paint inside the exhaust stacks and streaks on edges of stacks.
Combining oil and rust gives "orust"
"Orust" is made by adding a few drops of red, blue and black to the basic "Rust" color. This should be fairly dark and is applied to the wheel sides and to the metal sides of the trucks. Wheels and metal parts tend to rust quickly and oil/grease leaks onto them. The sideframes shoulds be painted with a color of the original truck(black or silver) and weathered with "Grime" or "Grust" as outlined above.
Notice that none of these colors are opaque, all are somewhat transparent. This is acheived by mixing the basic color with equal parts of thinner and allows the original color to show thru.
There are several methods for applying weathering, hairbrush, airbrush, or using some method to remove some of the paint.. Any method that will apply the paint can work. Often a combination of these methods will result in more authentic look.
A hairbrush is best used to apply "streaking". These streaks are often found at locations where fluids would leak and run down. Under battery boxes, at bottom of hood, bottom of air intakes where oil might leak out and "streak". Another spot is the fuel tanks where fuel is spilled or leaks due to a loose or poor fitting filler cap. Occasionally the fuel gage will have a small leak.
An airbrush is best used to apply overall weathering. Overall weathering is what happens when the locomotive is operated over the road and rain, mud, grease and diesel "soot' accumulate over the outside of the body. Generally the trucks will acquire a grayish color due to brake dust and liquids splashing onto the trucks. Don't forget the brake piston linkage will remain shiny. One place the airbrush is superior to other methods is on and around the air intakes and exhaust stacks.
I don't think there is more to add without an actual workshop.