Front left wheel arch. The inner panel was much worse than the outer one. Dirt and condensation drip down into that place, and have no way to get out. Cleaning is impossible because there it no way to get there: you would have to go under the bonnet, behind the battery, over the inner part of the wheel arch, and then down. You can't even see there, let alone cleaning it.

Note the new prefab panel waiting on the floor.