I wire brushed inside/outside the wing and the lip is surprisingly still good, albeit the surface pitting.
My plan is to paint with metal primer then paint over with smooth bitumen which if applied up to and along the arch profile lip on the outside, will look respectable.
When Mr Rust comebacks and it gets too unsightly, I'll get a new wing, paint and fit. With fasteners greased up, about an hour's work for off with the old and on with the new.