Easiest way to explain it is for you to go to the Library section of the site and read through the article on
- Undercarriage - Finishes and Sealer Guide for 1966 San Jose Built Mustangs & Shelby'sA couple of screen shots of a couple of pages from that article
On the main page down toward the bottom you will find a main section titled Library then fine the year your interested and then the articles
There are allot of details to include in a thread so that is the purpose of articles. There are numerous threads showing examples for how San Jose did these things in 66. Both in the 66 sections and the unrestored picture threads.
If you don't find exactly what you need or want a clarification post a follow up and we'll assist
The tint of red oxide for the firewall rearward (a different source for the firewall forward was used) depends often when the car was built at San Jose but most of the year IMHO the rear section was more on the muddy brownish tint of red oxide. And remember red oxide does not mean open primer that creates a flat look but instead an epoxy based product that created a smooth often slightly shiny look. More about that and products people have had success with in other threads