IMHO after-market rust protection is a waste of time and money.
Probably going to charge you several hundred dollars for about $4.00
worth of product.
I have not read Audi rust warranty but find out if any kind of
after-market stuff might actually void the factory warranty.
If it is just a spray on and not the kind that opens up a body cavity and
fills it the cavity with a spray, then I dont know how it can do much
good. If it is the open up a body cavity type I would bet that is a
no-no as far as Audi honoring the warrnty is concerned.
Spray on rust proofing might marginally improve sound
deadening--something an A4 does not need. About the quietest car I've
driven in a long time.
Save the $$$ and use it to have the car washed a lot in the Winter to
get the road salt off of it and you'll be a happy camper.
Bill Murin '87 4KCSQ