Well maybe 2. After cutting out the section of the old quarter, should I have the backside of the old panel also sanded down to bare metal at the weld joint? (I am going to butt weld the joint)Also, before I weld in the new piece should I be priming or painting the back side of the existing and new panel with something as I will not be able to get at it in places after the new panel is welded in place?

I really do appreciate all the help fellas

Thanks

I think you should be grinding to bare metal the front and back side of each panel, old and new, about an inch on either side of your proposed butt weld, so that old paint or powder coating (EPD) will not melt and flow into the weld joint. While you have the old panel cut out, this is the time to get in and sand and paint the remaining parts of the car that you can't easily get at with the new panel in place. I brushed on POR 15. I did not paint the inside of the new panel as the EPD coating looked thick and intact. I probably should have. I intend to scuff it with scotchbrite pad later, and when I prime the car, I hope to shoot primer and paint onto the inside of the panel in all areas I can access with the spray gun. I was told by others that the epd coating is not meant to be a final surface to be left for all time.

My rule of thumb is any metal with in 1-2 inches of the weld joint needs to be bare metal with weld thru primer applied. But in your case you are welding with the butt weld so no primer is needed. On my full quarters for the 70 before I welded them in place I used the red scotchbrite then applied one coat of PPG dx1791 self etch primer, then PPG dplf90 expoy primer and mixed it as a sealer with a slighty higher amount of reducer. The reduced expoy will sand easier so when the time comes for the splatter paint just scuff sand and shoot. Instead of expoy you can use 2k or urethane primer but I choose the expoy because its black, in reality the 2K is much better to sand