Gettin back to the original post, I agree, 1/4's are expensive, if you can save what have, its worth it. I also agree filler probably is not the fix for pin holes in that area, but Mig'in pin holes shut, is the most difficult things for me anyway. copper back or not, plus your welding out of position. I've seen on another site where they repaired an area like that by cleaning the metal, then treating with something like picklex,(rust converter/treatment), then used aluminum tape as a backer, and filled the front with a polyester, waterproof filler. There may have been other steps involved. The site is called autoybodystore.com. there are alternatives,you can also coat the back with a rust encapsulator. Guys on that site have had repairs last for over ten years like that.