.../...what color the underside of the body should be. I know the transmission cover plate is body color, but what about the rest of the sheet metal?
The whole body shell was dipped into primer - this is kind of grey shade.
Then a protective sticky rubberlike plastic material was applied. Then, car body was sprayed in the colour of the vehicle - this resulted in some "fog" of the finishing paint being sprayed from the sides coming in and gradually fading towards middle of car. This is true for all M-B of the sixties. On our W113 pagodas, the area under the doors have the black (deep grey) outer threshold cover plate. Under it, the body was painted black in a strip. Then, a protective wax was applied - basically transparent yellowish, but it has become brown with the years on any car you see today. Finally, if the owner chose to do so, the car could have been treated with a blackish underbody tar, as rust proofing
Below is a text by Vince Canepa of this Forum, which sums it up pretty well
/Hans in Sweden
Vince Canepa quote: "The entire bare chassis was dipped and primed. The primer is grey/green in colour. Then a plasticized (PVC) undercoat was applied. This undercoat was tan.
When the colour coat was applied the wheel wells received a pretty good overall coat, but the undersides (floorpan) received mostly overspray. If you view them from the outside they look body colour. If you view them from the center of the car, a lot of the primer or tan show through. The driveshaft tunnel, above the rear axle and any other areas that would be "shadowed" to a painter standing outside the car are almost entirely primer.
To get an idea of how it would look, a red car would have 80-90% colour coverage in the wheel wells and be pretty much body colour there. Underneath the car would see maybe 40-50% coverage and appear tan/pink from the effect left by the overspray. In the driveshaft tunnel there would be mostly the primer (grey/green).
/Vince Canepa"
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