If ally then oxidation... :-/ was hoping I might b able to cut/scrape back the affected paint and touch it in but I've only done that with steel panels so don't know if that would stop the oxidation. It's strange though as there is no break in the paint surface. I have to say the corrosion protection on the car generaly is rubbish. I dread to think what it would be like if I used it year round..
Yes I've had "spiders" under the paint on another car and they usually come from the edge of the panel, mine came from the sill edges upwards, is the bonnet edge scraped or rubbed at that point?
Our car's a bit older than yours and I can see what you mean about the corrosion protection. Visually, there's not much on the underside and the edges of the metalwork starts to look dry and flaky after a few years. Most cars are like that at 6 years old though.
After a good look round ours it was clear there was nothing serious and the car would most likely manage its 12 year perforation warranty ok. The paint warranty is 3 years for external surfaces. Trouble is that it's hard to go back after corrosion has taken place so we had it Dinitrol'd which is relatively cheap and should now extend the life to however long we keep it given that it's a w/e car.
This is a big improvement on two Mk2s we went to view which both had rust-through on the sills and wings at 8-9 years old.