I have reached the point of where I am content with all the details I have applied to the cars. Some I skipped because they will not be visible when the cars are running on my layout, and others I skipped because they were just too time-consuming to add or I don't have the skills (yet) to implement them correctly, but all have been evaluated. It is now time to focus on the next step: painting. I used blue masking tape to wrap the couplers, because I don't want them painted the body color, nor do I want to get paint in the moving parts of the coupler. I removed the trucks, for the same reason. I used a pressurized can of air to blow away all construction dust from the models. I recently learned from Model Railroad Hobbyists magazine that Floquil's "Primer" paint is not an actual primer, but rather the paint color that is made to look like primer. They recommended using Krylon's Primer spray can. I could only find it at Walmart.
Here is a close-up of the finished cars ready to be primed.
Spraying the primer went very well. These Krylon cans are high-quality ones. They have an adjustable nozzle for the type of spray you want, and they apply a nice, even coat of paint. You can apply a second coat within as short as one minute and they are dry to handle after ten minutes. I sprayed the top of the cars first, and let that dry for a while. Then I turned them over and sprayed a single coat on the bottom, directing the spray at an angle aimed at all four sides. Then, when that had thoroughly dried, I went back and applied some very quick sprays in areas that I had missed. I loosely placed the cars back on the trucks for this photo.
I used the same procedure for painting the body color, except that I used the airbrush. For paint color I used Floquil's Oxide Red. All the prototype photos I have are black-and-white, so I looked at what others have used for their model colors. I concluded that this was close enough.
I painted the underbody of the car the same color. From actual prototype flat cars I have seen in person it seems that they painted the entire car the same color (at least non-PRR ones).
Before re-installing the trucks, I painted the wheel faces with Floquil's Rail Brown. I tried to remove the weathering material that is used on these wheelsets to have the shiny wheel tread, but even after grinding away at it with the Dremel tool, the oxidation returned within minutes (this was both for S-Helper Service and American Models wheelsets). I gave up on doing that. I'll eventually replace them with NWSL wheels, whenever they will have them in stock again. The truck body I weathered, first by using acrylic white paint to highlight the edges (dry-brushing), and then by applying rust and gray weathering powders (I use Bragdon Enterprises' set). The truck now looks reasonably "used", although it is hard to see given the dark confines of the wheels under the flat car. It was a challenge just to get this photo of the truck!
The last thing I did with regard to painting was to hand-paint the air hoses. I painted the hose itself using Floquil's Grimy Black, and then I painted the gladhands using Poly Scale Stainless Steel.