Hi Jeff,
if I understood correctly first we apply red oxid primer (expoxy transparent primer and then colored to red oxid on my case), .................
Lets try it a different way
1- Fender painted red oxide at the stamping plant and shipped to the car assembly plant
2- Fender checked for straightness and corrected as need be then painted on the exterior surfaces with a light gray primer
3- Fender along with the rest of the painted parts not installed on the body were painted while on painting racks of fixtures painted with a light gray surface primer
4- Fenders and other parts were next painted body color paying special attention to the exterior surface applications and spraying paint during this process along and on to the wheel lip and rear surfaces that face to the interior of the car when the door is open. This is important to what you are doing since all the overspray (read "allot" since they were using high pressure guns not the ones we use today) from this application traveled to those surfaces directly behind those edges (wheelwell, front face and rear edges) producing a logical pattern.
5- Much later in the build process, second half of production, the fenders were installed.
6- Once the car was pretty much complete sound deadener was applied to the inner fender panel and front wheel wells over some of the surfaces. This could include the bottom side of the top surface (bottom of the top surface of the fender) of the wheel well in some examples but also around the front and rear splash shield to fender area as part of a later overall pattern to seal many of the attachment points and deaden the road noise and impact from things being thrown up by the front tires as well as other purposes