I have just had my front lower frame panel replaced due to rust...this was worse than I thought. The body shop used "Restoration Design" suspension pan. A major effort that improved the handling significantly.
This is the area under the battery which shows surface rust.
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This shows the bottom where the front suspension connected.
I was really unaware this was so bad until I was in a small accident and the front suspension was exposed. The rust was covered by the undercoating and was not really visible. The strange thing is the handling deterioted so slowly that I didn't really notice much except understeering and some pulling on braking. During the last few months I was taking it easy until I could get the car fixed.
This is also an '86 Carrera so I thought that rust was not as big a problem with the zink coating. Shows that if you see surface rust you should check closely.