In the rear wheel arches, there are metal lips which were meant to be mounts for the inner plastic wheel arch covers.

However, as far as it is known, Suzuki has never produced those inner plastic wheel arch covers for rear wheel arches.

Therefore, these inner metal lips just accumulate mud and grit above them, which is quite difficult to flush out. Eventually the area will rust.

It is recommended that you either grind these inner metal lips off, or bend them up and fill the space above them with some filler mass (thick bitumen, silicone, whatever). That will prevent the mud and grit from accumulating in your rear wheel arches.