These cold days makes me go through the internet in my search for different stuff. I found a great picture showing how the undercarrige looked after it was painted in primer. The body was oversprayed and when it came to the drying procedure you can see evidense of the paint drips.
The primer coat on the bottom was NOT sprayed. These car bodies prepped in a 13 step process:
1.
2. Each body is dipped and sprayed in hot alkaline cleaner.
3.
4. Body is dipped and sprayed in hot rinse.
5.
6. Body is again dipped and sprayed in rinse.
7.
8. Body is dipped and sprayed in a paint-anchoring phosphate solution.
9.
10. Body is dipped and sprayed in cold rinse.
11.
12. Body is dipped and sprayed in special solution to further aid paint adhesion.
13. Body is dipped 22" deep in special corrosion-resistant primer to completely coat all critical lower body sheet metal and fender peaks inside and out.
After dipping the body, the primer is oven-dried to ready the body for painting.
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The above description is from factory literature contained in the book, "A New Kind of Plymouth: Barracuda 1964-1969 A Book of Information".
So the drips on the bottom of the car are NOT from spraying, but from being dipped in a very large vat of primer suspended from spreader bars that appear to have hooked onto the underside of the roof. I saw these "stalagnites" on the underside of my 69 cuda. You can see the level to which the body was dipped by looking at the interior panels inside the rear quarter panels. The metal above the dip line is usually rustier than the submerged part. The actual color of the primer is kind of a dark muddy gray and any body color on the underside is from overspray that happened when the finish color was sprayed on. No attempt was made to completely coat the underside in shiny body color that is typically found on over "restored" cars. A correctly restored car would not have shiny body color over 100% of the underside.
In my experience, the anti-rust dipping may have been good in theory, but actually it did not seem to prevent rust, only slow the process down a little. Around here, it was common to see rust holes through the bodies of cars after only 4-5 years on the road.