Hahaaa, now I know why you commented on the tips of my S8 exhaust when I shared the photos, it all makes sense now!
Mine wasn't as bad as yours when I got it from A8 Parts but I only spent about an hour polishing the tips..........using the same type of kit you tried (with excellent results) the second time around.
Nice job Oli, looking great
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Audi D3 S8 - 2007 - V10. B&O, Phantom Black, black and silver interior, and still completely standard with everything working as it should
Audi TT - 2015 - Ultra