Yeah, just realised when posting. I’ll have to go and take another, focussed this time. Although I say it’s temporary, it’s only ever getting done again if the arch bubbles - which it shouldn’t. I recently cleaned the lip on the inside of my arches. Go to your rear arch, and at the inner top you will feel a fairly significant lip, which on my car was holding a lot of dirt, and I imagine, if left, would result in corrosion.