Nick, as you say you have rust appearing they must be steel, I'm quite surprised really I would have thought aluminium and the outer edges rolled over spring steel is the normal corrosion point on these pre superform wings it is caused by electrolytic action between the dissimilar metals being in contact with water and possibly salt. It will have a white powdery appearance where the corrosion has started.
Replacement for superform is the absolute purist answer this also preserves the value of the car.
Fibre glass replacements are available and I once viewed a White Plus8 with them and visually they were very good.
Hamwich on here recently completed a great job on his car by replacing all four in fibre glass and it really looked great. There is a thread somewhere on here.
I think the general consensus is repair is a temporary and quite costly exercise.
