I used the same brushes and paint for these next steps as I did to weather the sides. Below is a picture of the brushes and the paint.
For the top, I mixed the same Liquitex Ivory Black and Titanium-zinc Everwhite oils with a little bit of Winsor & Newton burnt sienna. I thinned this into a wash with odorless turpenoid and the applied it to half of the roof with the brush on the left above. I made sure to get the wash under the running board and around the sides of the hatches. Next, I took the brush on the right above and began stippling the wash. As the brush accumulated paint, I would wipe it on a paper tower. Every so often, I would dip the brush into turpenoid and then wipe it almost dry on the paper towel. The photos below show the results.
The most important part of this step is to soften any sharp edges by continuing to stipple the wash very gently. It takes a bit of time but the results are worth it. Adding a very little bit of turpenoid to the brush can help loosen any of the wash that has begun to set up. Also, be sure to stipple the areas around the base of the hatches. The wash tends to accumulate here and needs to be softened up.
To add some additional color to the roof, I mixed some more of the burnt sienna into the wash and then gently stippled this on around the roof hatches to represent additional grime and rust. This wash was also applied to the running board at this time. The photo below shows the results.
At this point, I sealed the weathering on the roof with a light coating of Dullcote. Once this had time to dry, I started on the ends and outlets.
The first step on the ends was to make up a wash similar in color to the one used in the second step on the roof. I stippled this very lightly on each to represent the grime that gets thrown up from the wheels. Next, I stippled the same wash on the bottom of the hopper bays on each end where the wheels would throw up grime. Lastly, I stippled this wash randomly to the sides of the outlets. The photo below shows the results.
After another application of Dullcote, I started on the trucks. I mixed up the same wash used on the sides and applied it to the side frames using the brush on the right in the first picture above. I wiped the brush almost dry on a paper towel and then stippled the side frames. Once this had dried, I added some AIM weathered black powder around the journal boxes. Next, I mixed up some artist's acrylic ivory black and Windex and added some AIM weathered black powder. This was applied to the wheel faces and the coupler trip pin.
The last step will be to add some more rust/grime around the coupler boxes and inner wheel faces and axles. I'll also hit the coupler with some weathering. Stay tuned.