The drivers side rear quarter panel, I discovered, has been replaced at some point in the past. It was tacked into place along the door sill, but the bottom of the panel would move outwards as the tack at the bottom of the sill had failed. There was also a patch of rust at the top of the panel which I had sanded and filled with body filler about 3 years ago. The rust had returned so I decided to cut it out and weld (first piece of welding on the car) a new piece in.
The rust line is where the panels had been joined by someone else 
The bottom part would ‘pop’ off the sill 
First cut (eventually square) 
Removed the seatbelt anchor bolt, handle & inner panel 
Yes, not the best work. First attempt! 
After a number of grinds, welds, grinds 
Coated in rust inhibitor 
The welds are pretty horrendous. I was blowing holes through the thin metal, filling, grinding, blowing holes. Eventually I had something that was good enough to have filler applied over it. As far as first attempts go, it will do. I’m most pleased about the spot/plug weld at the bottom of the panel as there is no longer any panel movement. It will now stay in this state until I’ve done the floor support and floor pan. Then I’ll spread some filler…
My Mustang Journey 