Fender apron patches - a typical vintage Mustang trouble spot
There are two apron overlaps on each side...and both need to be repaired. This all happens in the engine bay. Once the fenders are removed, you can see there are three pieces of sheet metal that make up each side of the engine bay - the front apron, the shock tower, and the rear apron. The shock tower is the middle piece. It's thick metal at 14 gauge. The aprons in front and behind are made of thinner 19 or 20 gauge metal.
Where the apron meets the shock tower, the metal is just overlapped and spot welded. The only thing that was used to protect against corrosion was hope and a short warranty period. The seams between the layers of metal are open on both top and bottom, so any moisture from the engine bay, or - more likely - the wheel well, will get in there and just rot the metal away.
Doing this repair, you get to learn how to drill spot welds, butt-weld different gauges of metal, and then use plug welds to join the two pieces together to simulate the spot welds from the factory. In fact, I'd recommend this as a good first welding project on a car to get a feel for how it's done - the metal is basically flat, and mostly unseen, so you can relax a little bit. Very early on, when I was figuring out what I was getting into, I read about this in detail on this blog by a hoopy frood named Alex, and it sort of gave me confidence that I could successfully do this, and other work, on this car. So I blame him.
I'm only showing one side since the other side is a mirror image. Plus, I'm about to discover I need new aprons, shock tower, and frame rail on that side anyway...