Hey Y’all;
I don’t understand the want to use sandpaper, even fine grit, for the car roofs; passenger cars had long had metal exterior roofs by the era of this car style.
I did a lot of custom painting of N scale cars in to N&W for a few N-Trak fellows in Va Beach Va in 1980s, and Jack had some slides of N&W cars in Roanoke in the 1950s, at least one with the cars seen from above. Thing to do might be use flat black paint with repainted areas of fresher satin black paint, and a few quick and dirty, rough textured, reseal and patch paint spots here and there. A couple of them we classed as downright scuzzy.
Here’s an on-line illustration of what that kind of appearance is http://www.railpixs.com/nw/NW%20commuter%20train%20car%20at%20Chicago_Sept%2077.jpg
Blue one showing some rust http://www.railpixs.com/nw/NW_coach_RoanokeVa_Oct77.jpg
Lighting and film grain give there PRR car roofs a more uniform look, http://www.railpictures.net/viewphoto.php?id=257515
Here, cruise through here looking at PRR passenger car photos, some will be similar cars in color, http://www.northeast.railfan.net/rolling37.html
Note slight shading over where roof sheets are attached to carlines http://rrpicturearchives.net/showPicture.aspx?id=3801551
One more place to browse passenger car pics, http://passcarphotos.info/
edit: hmm, don’t know what it is going to look like to you, but here the word spacing has a somewhat random appearance, odd.