I had my '94 Ford Ziebarted right after I bought it. No rust of any kind yet and that was 11 years ago. The way my plan worked was that Ziebart would re-inspect and respray for free every year (if needed) as long as I brought the vehicle back in before the anniversary date of the initial application.

I moved to a different area a few years ago though, and haven't had it rechecked since then. But still, no problems. A few years after the initial rust-proofing application, I also had the sound-proofing spray-on material applied, and it did quiet down the road noise a bit more. Reflecting back though, I'm thinking that the rust-proofing material alone would have probably been enough.

I'd be curious whether Ziebart offers any type of rust neutralizing application prior to the regular coating of rust-proofing material. I've been under the VX a few times since I bought it doing various work, and it seems that the underside of our vehicles seem to start rusting kind of quickly. A comparison that comes to mind is the way the finish of most tow hitches I've ever seen tend to start rusting through their finish after being exposed to the elements. Now that you bring it up, I'd like to do something about the rusted areas I've seen before they progress any further.

Overall, I think it just depends on the type of rust-proofing that was applied at the factory as to whether you need an additional aftermarket application. Some chassis and body parts are completely dipped in a bath during the assembly process, and if that doesn't take care of rust problems in the future, I doubt that much else would. The Ziebarting was worth it in my opinion though.