Up here in Canada...more specific Thunder Bay, Ontario (above Duluth, MN, along the shoreline), we have a company called "Rust Check". I have an 08 HHR and have used them every year - no rust. My father had an '86 GMC 1/2 ton and he did the same - no rust....we just sold the truck in 2008 very minimum rust. I swear by this companys product. I will continue to have it sprayed professionally by these guys for the first 5 years then every second year from then out. And it has no conflict with warranty! For preventative measures...the $130 Canadian dollars every year I spend, is OK to me to keep the rusties away from my baby!

 

Really easy to do... We do that each fall before it starts to snow (before salt hits the road) with anything we're driving in the winter (and my truck for good measure). Have a can with a pump handle thing on it, and it shoots the oil across the room, so you know it's really getting up there. Squirt it in each drain hole, and anywhere you can, along fenders, in the hood, and let it run out for a week or so. Nice coating for the winter!

 

I have used a cheap spray gun from from Harbor Freight to spray used Tranny oil on the underside and a small gun to shoot in the door drip openings.

Any more I have not done it as much as I have seen while it may help a little it still ends up about the same after 10 year. Todays cars have enough undersider coatings on them that only the brake and fuel line for the most part show rust. The pans really so not have issues.

 

mine is a tx car and has been all its life and I have some rust on my rear fenders and it is a 07