rear wheelarch woes

[Perseverant]

Cleaning up under wheelarches with the intention of redoing underseal, the angle grinder wire wheel went through the arch on the driver's side. Bah! Anyhow, I've cut out the worst of the rust and a section of wing to gain access to the inner arch, which I'm patching up. I've also made an edge section for the wing, which looks like it will fit not too badly. I had the same trouble with our previous MX5. Good little cars but thank heavens I can weld faster than they can rust. This corrosion seems to have started in the lip where the inner and outer meet and are spot welded together - a common source of rust. I wonder if anyone has any bright ideas on this?