Daniel,
As best as I can figure it, they were not painted. Well unless they were with over-spray. They may have gotten under the car to paint the metel items under there, but I don't think they made a great effort to paint the rest of the under side of the cars. Also in the interior, not only was it not likely to have been painted but they used disinfectant after the stock rushes were over. This appeared white in color and can sometimes been seen dripping over to the outside of the car. Also while the roof top was painted, the roof walks were often left un-painted. The reason I was told for this was Painted Wood in the rain is very slippery. So to avoid injuries they left roofwalks un-painted.
Hope this helps and I'm sure others will jump in and offer other tid-bits
Kevin Cook