This may interest you. I received it from Lee Turner whose dad was a superintendent at the LV Sayre shops. As Eric mentioned, it depends on the time frame your modeling.
When these cars were rebuilt in the 1930's and received the Zenith or Viking roofs the roofs were painted. I base this on the limited pictures I have seen that showed the roofs had remnants of paint around the ribs and under the roof walk. That includes the remaining car in Coplay that Jim Mucka took close up photos of the roof in the 2000's. When the cars went through the post war shopping and received AB brakes and steel doors they were repainted again and on this second shopping I don't think the roofs were repainted. Cars Photographed in the 50's often show peeling and exposed galvanized roof sheets. Also in talking to my father he told me about freight car painting at Sayre at least and looking at the facilities at Packerton I'm sure the procedures there were the same. Freight cars were painted outside with a crude paint sprayer made from a length of lead pipe with one end hammered flat which was the spray nozzle and the air line and paint siphon hose attached to the other end of the pipe. The siphon hose was put in a 55 gallon drum and the airline hooked up and they basically hosed the paint on the car. No scaffolding was used so the only paint that ended up on the roof was overspray. This was such a crude method that runs and drips were everywhere but the cars did get a very thick coat! I asked my father about the roof and he told me that the railroads realized that Galvanized metal would not hold paint so why bother? You couldn't see the roof from the ground anyway!