The cement cars got dirty overall, of course, and a good start to the process is a light to moderate “dirt” color (my approach is the usual acrylic wash, colored with a mix of Burnt Umber, Neutral Gray, and Ivory Black; for more, you can read the joint handout by Richard Hendrickson and myself, available at: http://modelingthesp.blogspot.com/2011/10/weathering-clinic-handout.html ).
Once the base “dirt” is in place, some amount of spilled cement needs to be added, ranging from just a modest suggestion, to serious spillage. In later years, these cars sometimes had quite a buildup of cement around hatches, and with natural moisture available, this turned into lumpy patches of hard cement. In my modeling era of 1953, however, spillage was still on the lighter side, so I have limited myself to staining and streaking only.
Natural cement is light gray, and this color might seem like the best choice for showing cement stains. Indeed, these cars for many railroads were painted a gray color which was intended to show as little of the cement spills as possible, but in fact the cement streaks typically looked lighter than the car color. Accordingly, I believe a somewhat lighter “cement” color should be used in the weathering process for these cars.
Here is a prototype image, taken a long time after the period I’m modeling, and naturally I don’t wish to show quite this much cement staining. I took this photo at San Luis Obispo in 1992.
Note that the original lettering and SP emblem on this Class H-70-10 car is still visible, although the car has been renumbered (this was the only class of open-side covered hoppers on the SP).
In indoor lighting on a layout, it is often necessary to exaggerate effects somewhat, and I believe that cement staining is an example. In my acrylic method for these cars, I’ve mixed Titanium White with Neutral Gray, to get an intermediate color, followed by some very minor highlighting with straight white. I have weathered a number of cars but will only show two.
First is an SP car, which is an InterMountain (IM) model of SP’s Class H-70-8. It’s photographed alongside the depot at Ballard. The car is barely two years old in my modeling year, so has modest cement marks.
I also added some cement staining to an IM model of the early Santa Fe cars with external roof carlines (Santa Fe Class GA-65). It is also in Ballard.
On this car, the freight car red color shows the staining far more prominently. The two cars together look like this, with a more overhead angle.
This post is only intended as a suggestion of a way to weather cement hoppers, and your choice may be to do heavier or lighter suggestions of cement spills. But whatever you do, cement hoppers do need at least a little of this characteristic staining, even when relatively new.