This is the lower part that is behind the front wheel in the wheel arch, this part is also called the rear part of the wheel arch.
Originally, this sheet metal part was intended to be used as a repair sheet to be placed above the existing sheet, but today it is mainly used as a restoration sheet.
The rotten sheet metal part is removed and this repair part or part of it replaces the existing sheet metal. If done properly, you get a seamless repair, so no rotten sheet metal is covered and you get an invisible repair.
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