After washing and drying the model, I painted the underside of the car with Floquil "Grimy Black" using my airbrush. I let that dry for just an hour, and then went back and applied the Floquil "Oxide Red" body color also using the airbrush. The photo below is of the car just minutes out of the paint shop. I had some trouble with the airbrush, and I also had my first major accident with the airbrush. I had to use the cup for painting and some of the paint splashed out of the cup onto the model. I tried to fix it as best I could, but I'll have to hide some more of that with judicious weathering.
For decals I bought two sets (one for back-up) from Des Plaines Hobbies. There are two sheets of each in one package (one of each is shown in the photo below). I was hoping to model PRR #65400 since there are quite a few prototype photos of it available, and it turns out that the decals have that number (see the lower left corner of the second decal sheet). The decal has a built date of 12-39, and the appropriate X37 class information. The decal sheets also include information for the PRR X37b, X43a, X43c, and X46 boxcar classes.
After the paint cured for a few days, I sprayed it with Testors Glosscote. I let that dry overnight and then applied the decals. I had a bit of a hard time with the decals. I don't know if they were old stock, or what, but they didn't settle down very well in some areas using Walthers Solvaset, even after poking holes in them.
The PRR also put the car number on the car's ends.
Before I started to weather the car, I wanted to install the couplers. I used my standard Walthers Protomax HO-scale couplers. I had to tap and drill a hole for those couplers (and cut off the side mounting holes from the coupler box). The photo below shows that the top of the coupler sits too high, by the thickness of a piece of styrene (I believe it was 0.030" thick). I then removed the coupler, cut a piece of that styrene to match the size of the coupler box, and mounted them all back in place (the styrene was glued to the coupler draft box with superglue before installation).
Before I applied weathering, I used some "chalk marks" dry transfer decals from Clover House (part #9911-01 for S-scale). I normally don't like dry transfers at all, but since these chalk marks are usually temporary on the prototype, if the decals don't completely transfer, that's OK. This was my first time using the decals and I am quite happy with them. I plan on adding them to my other cars too. In the photo below, you will see "Pgh" to the left of the car number decals, and some scribbling to the right of the door. I applied two different decals on the other side of the car, in approximately the same locations. The final step in finishing this model was to apply some weathering powders (I use Bragdon Enterprises powders). For me weathering is more of an artistic endeavor. I just "go with the flow" until I find something I like. This side profile (and it is a bit hard to see under the layout's lighting) has weathering applied along the bottom of the sides using various colors, weathering down the rivets of the side panels, and some rust weathering here and there. I highlighted some areas with dry-brushing white, such as the door details and some of the ladder rungs and grab irons.
On the roof I used a light gray powder to give the raised roof panels a sun-faded look to them. I also highlighted the seams by carefully dry-brushing white paint on them. I also did that to the edges of the roof, to simulate some paint wearing off. All in all, this was a fun project. It took me about a week and half of mostly evening's modeling time to build the kit and paint it, so it was a relatively easy project to do. I wouldn't mind doing another one of these cars, but then make it even more accurate.
Update: On a whim, I decided to intensify the weathering on this car a bit more. I used a rose-colored Bragdon weathering powder to highlight the panels' seams on the sides. I also applied that same powder along the bottom edge of the car. I think that gives it a nice rain-water streaking effect.
Although I had weathered the roof, taking this car to the local train shows and having to brush off the dust that inevitably settles on the car's roof, the previous weathering had been mostly removed. Also, the roof still seemed too shiny for my tastes. I used that same powder to cover the entire roof. The white of the roof seams still shines through (at least in person), but the roof looks more aged now.