I know this topic has been discussed may times before. While scuff sanding the underside of my 67 NOR 5B convertible I see the whole underside is covered in what I thought was butternut yellow paint. But what is confusing is this appears in the upper parts of the transmission tunnel too. I have researched and read the articles about the underside colors and the finishing process on CRG, so the underside is said to be anywhere from red to black to gray but I find it hard to believe the underside would be completely painted in the body color considering the painting was automated (minus blackouts). What I have is not just overspray because it is too uniform and applied on purpose and on areas outside the robotics reach.
In the attached picture (Pic 1) the underside layers are as follows from bottom to top:
raw metal --> red primer --> yellow coating (paint or primer?) --> Rustoleum I applied in the 80's.
Looking at the topside body color picture (Pic 2) the layers are from bottom to top:
Raw metal --> factory gray primer --> factory butternut yellow paint --> (repaint) gray primer applied in the 80's --> paint applied in the 80's
The picture with the paint sample (Pic 3) of the 80's butternut paint job was scuff sanded the same grit as the underside to remove the gloss. As you can see it is noticeably lighter than the yellowish hue of the underside as well on the chassis rails that are completely opposite where the robotic sprayers overspray would hit.
If this isn't paint then this leads me to believe that a yellowish primer was also used, possibly a form of dichromate?