Re: sheet metal
What I know.
GM used a primer gray (2k my guess) to seal the insides of the fenders. My GMC syclone original fenders had this gray "dipped" primer. When I say dipped, Its like GM or who ever had the job was to fully submerge the fender to completely seal insides. Though I may be wrong..
Some one backed into my truck and I was faced with replacing a fender. I noticed on the inside the full-seal-gray with factory black overspray. I am sure this can or will vary vehicle to vehicle as all are different colors. I just noticed the primer with BLACK in my case (SYCLONE) overspray in the oil cooler area. there is webbing in the fenders that created the spray. I tried to replicate this in my vehicle and to me, This is one thing that is easily pointed out if you are looking. I went through the trouble to get a BLACK oem fender and it needed work I tried to preserve the inner primer finish and could not.
You can look at my passanger side and it is very clean, gray primer is visible and the minimal factory over spray .Where my driver side is a different story