I am finishing up on the metal work stage of my '51-3100 restomod and will soon be looking at body work and painting. My plan is to media blast the sheet metal and primer it prior to body working to protect the steel. I have researched both epoxy primers and self-etch primers, both having their advantages and disadvantages, sandability, and compatability with bondo and hi-build primers. Any recommendations/procedures for either method for a beginner body worker would be appreciated. Any recommended publications on this subject would also be appreciated.
Thanks!
Back in the late 70's to early 80's the auto manufacturer's were experimenting with galv type coatings to help combat the rust issues seen. On a PH level, these type coatings are slightly alkaline (base) and the normal primers used were ever so slightly acidic to promote adhesion. Combine the two together, and the base helps to neutralize the acid in the primer for less adhesion. (this same scenario causes adhesion issues when using Soda Blasting without properly neutralizing) Ever see pickups with paint peeled off in sheets? So their answer was to add more acid into the primer, hence the etch primer. The collision industry saw it used for many years afterward to insure the paint repairs would adhere to the factory metal. Today's autos are using more of an e-coat dip to seal and rust protect the body, so the etch primer is less used today. That, and they are finding that too much acid, if trapped under the paint surface, will cause a reaction, especially when exposed to heat (think car shows and hot sun) and will cause adhesion failure as the acidic process trapped under the paint releases the outer paint layer.
For rust preventative measures, and given a properly prepared and abraded panel, NO primer can match the properties of Epoxy Primer for preventing rust creep under the paint, to include many of the snake oil POR type products. That's not to say epoxy can't have issues though.... As mentioned, any residue from soda blasting (base) will have epoxy primer (slightly acidic) or most any other substrate falling off in sheets. IMO stay away from soda blasting to eliminate this issue, and use Epoxy Primer for the best rust preventative measure going today. Epoxies aren't used by the car manuf. as they take too long to dry as compared to E-coat. The factory processes are less about what is the best way possible and more about what gets this vehicle to the end of the line quicker.
