Originally Posted by epGTO60
From my family's experience living in the snow belt in Ohio and rust proofing cars, it actually tends to do the opposite. After 8+ years you end up with bad rust because the rust proofing ends up trapping moisture against the metal which eventually turns into rust and progress quickly to rotting out panels. My family used to get the rust proofing reapplied every year but it can never completely seal everything so it doesn't work. It tends to hide what is really going on underneath.
My family has not used rustproofing since our 99 GC and our vehicles have lasted a lot longer without rust. My family tries to keeps each vehicle for 15 to 20+ years so we have seen the difference.
With the way the manufacturers treat the bodies at the factory with anti-corrosion dips and such, you are better off relying on that and keeping the underside clean. As others have said, periodic car washes with chassis baths during salt season and then an extra good cleaning after salt season goes a long way.
I'm not familiar with the type of rustproofing that needs to be reapplied every year. To me, that sounds like a wax based product. I'd have to question how effective that type really is.
The stuff I had applied to my vehicles was a black, tar-like petroleum based product... kind of like what Ziebart used in the 80-90's. There were 2 kinds of this type of rustproofing... one that dried like paint and the other that stayed tar-like and tacky. I used the one that dried. The only way that stuff would trap moisture against metal was if someone was stupid enough to apply it when the vehicle was wet.
That stuff wasn't cheap to have professionally applied... it was in the range of $500 back then. It stood up perfectly to anything thrown up underneath the vehicle. Never had any problems with rust starting anywhere.
Rustproofing is only as good as the guy who's applying it. Anyone can run a spraygun. It takes attention to detail to ensure that all the seams, nooks and crannies are covered properly.
Like I said several posts back, I haven't had rustproofing applied to my last couple of vehicles and the only place I would recommend it on a new vehicle today would be inside the doors because that's not an area that you can easily get into to clean.
My 02 went through 12 winters worth of road salt. The underside stayed remarkably rust free but I started having trouble with rust breaking out along the bottom seams of both rear doors... from the inside out.