The rear wheel arches have a lot to do, especially the lower parts closest to the road. You can see this clearly during snowfall or on muddy roads. Dirt sticks to the bottom of the wheel wells and if you don't remove it immediately, it provides a breeding ground for rust.
During restoration, you'll find that new sheet metal is needed to repair the damage. In the front, this is a piece of sheet metal that was originally part of the wheel well. The right solution is to grind the defective part and weld it seamlessly to the new one. At the rear, it's a spare part that replaces the old one, you have to drill the welds by points, remove the rust and put it back the same way.
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