It is true that rust prevention is much better in vehicle construction than it used to be. However you should not rely too heavily on the new car warranty from the vehicle manufacturer. Holes gapping through your body sheet metal, is the only metal and measurement that is important to the rust perforation warranty of the new vehicle. Holes and rust in the underbody, wheelwells, trim connections, and surface rust (or rust coming up to the surface from within the vehicle) without a hole poking through are not covered by warranty.
A good rust control solution can help extend the life and quality of a vehicle into its old age. To retard the inevitable onset of rust, the best product is one that is hydrocarbon (oil) based, and is able to spread into unseen crevices and inner walls of metal forming your vehicle's body.
A good rust control product must have characteristics of:
I should point out that I do not work in an automotive related trade, nor am I related to anyone who does. I do not benefit in any way from sharing this information - it is done in same spirit as the postings I have made to the rec.auto.tech and rec.auto.misc Usenet newsgroups - advice for those seeking it, based on what I have personally learned.
The APA (Automobile Protection Association), recommends Krown Rust Control for rust protection. Rust Check sells a very similar product/service, but the APA was not completely happy with their warranty backing. The APA is an independent and member funded consumer advocacy organization in Canada, concerned with the quality of product and service in the automotive industries. The APA publishes the newsletter Lemon Aid. I believe Krown is established in Canada, but not in the U.S.
Both Krown Rust Control and Rust Check use a wet hydrocarbon type of spray, mostly through existing points of access (take out the taillights, etc.). The mixture is very fine, and travels as a spray and mist into areas of the vehicle body that light never sees. This retards the onset of the most insidious type of rust: that which comes from the inside out. You have probably seen cars with paint bubbling, and when the paint is scratched off, there is rust underneath. This is often caused by rust starting on the inside of body panels, rocker panels, etc., and eating its way outward. When your vehicle goes through temperature changes it can cause condensation on the inside of the vehicle, and this moisture can accelerate oxidation.
The charge for the service should be in the $100 neighbourhood. Krown offers a 10 year warranty if you bring a new car to them within a short period after purchase. Otherwise the used car warranty of 5 years applies. It requires an annual respray to maintain this warranty and locked-in pricing.
The hydrocarbon is specially blended to produce these qualities: high spreadability, displaces water from seams and crevices, does not deteriorate rubber and plastic, does not block drain holes, non-toxic (Krown's at least). Any holes drilled should be limited to metal that is protected from direct exposure to the elements, such as inside the trunk or inside the door.
The application of the product is just as important as the product itself. Try to find someone that is going to apply the spray with some care and knowledge. Just as there are young monkeys working in the oil change shops there also tends to be inexperienced (very often seasonal) help in the rust control shops. Finding the owner applying the stuff is potentially a good thing.
Before I went to Krown, I tried a garage that used some no-name anti-rust wet stuff. After the guy was finished it looked thorough enough. But then I took off the "inspection plate" on the tail-gate of the pickup and this allowed me to see all of the hollow interior of the tailgate. There was plenty of goop right at the drain hole points (presumably where he placed his applicator), but everywhere else was dry.
Here are the problems with other types of anti-rust solutions:
I have performed a little test that has protected a ball of steel wool dunked in Rust Check and then tortured with alternating water and air, and sometimes salt water. After a year of being exposed to a situation that would have turned it orange in 24 hours without protection, the steel wool was still pretty much in original shape - just a little orange tinge in the water, and very little loss in mass. It was doing very well (shiny) until I added salt to the water at 8 months. The control group steel wool didn't need salt to make things worse - it became an unrecognizable mass of dry grey dust with an orange crust a long time ago. I couldn't keep the control group steel wool emerged in water because the oxidation consumed water quicker than I could remember to fill it up. I did not give the protected steel wool any more protection than an initial coating in August '94. Of course cars are made of metal better than steel wool, but if a product can protect steel wool it should have the capacity to do an excellent job with metal having higher rust prevention qualities.
As many people have pointed out, if you live in the south or where road salt is not used, or you do not plan to keep your vehicle more than 5 years old, then rust control is probably not necessary, although it may increase your vehicle's resale value.