Rear quarter body help needed

GeorgeH said
One of the things most of us fussing could probably agree on is that if left unwelded the exposed edge seam that runs down the side of the car will crack the filler over it sooner rather than later. Some extra time now will save some headaches later. It might last a year or it might only last until the car gets flexed.

And that is exactly why you don't put the weld on the backside of the flange (and on the inside of the quarter) like someone else suggested. Because that leaves the un-welded edge of the flange seam under the bondo on the outside of the quarter to crack and absorb moisture, while the smooth inside hides that method of repair so the next guy doesn't know he's sitting on a time bomb.

GeorgeH said
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I get that, but it doesn't make a lot of sense to spend hours blocking filler, primers, prepping and painting over that and the $$$ associated with the materials. A long afternoon could have the panel out, glue cleaned up, refitted ,welded, and ready for filler. Back where he started but in a little better place. Easier to take care of it now instead of a week after the car is painted and hits the road and you notice the gel-coat like stress cracks

Exactly, even with as much work as it will be now it will still be easier than waiting. Paint it now, and later if you ignore those stress cracks when they pop up (and they will) rust will start to form in the seams where the adhesive was burned off by those spot welds. It's not going to get easier to deal with, just harder.