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New-car buyers have invariably heard sales pitches for extended warranties and protection packages that have become the bread-and-butter of many dealerships.

The most commonly recommended add-on is rust-proofing, but its value is heavily debated.

"Today's vehicles are manufactured with good corrosion protection … rust problems have almost vanished in modern vehicles," Consumer Reports says its annual auto surveys show. "Standard rust-through warranties for most domestic and imported vehicles run five years or more."

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But with consumers often opting for extended-term car loans that can last up to 96 months, a small investment to keep rust away may be a worthwhile consideration. This is especially true if you live in Central or Eastern Canada, where winter salt is a major cause of corrosion.

Not all rust-proofing methods are the same, and prices vary from about $100 to more than $1,000. The key to avoiding a scam is to understand your options and to ensure you are getting a fair price. Here is a rundown of the most common methods:

Electronic module

The latest technology in rust-proofing involves installing a small device in your car, known as an electronic module, which then sends a weak electric current through the metal and in theory stops it from rusting (corrosion is an electrochemical reaction).

Most car dealerships have been moving toward offering these modules because they are easier to install and typically fetch a higher price tag, which can range from $500 to $1,000 at a dealership. However, you can easily find similar products outside the dealership for less. Canadian Tire, for example, offers a module for $300 plus installation for $60.

The science is still out on this method, with mixed reviews coming from both experts and car owners. While the CAA recommends rust protection to prolong the life of your vehicle, it prefers and endorses sprayed solutions instead of the modules. The only module Canadian Tire offers holds an average consumer review rating of 2.7 stars out of five on its website, with more than 40 reviews at the time of writing.

Tar-based spray

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The tar-based method is also known as undercoating. It involves spraying a black, tar-like substance on the floor pans, wheel wells, and other exposed parts of the underbody of your car, which then hardens and acts as a permanent shield against moisture, salt, and other elements.

This will not protect the whole body of your car though, and it cannot stop corrosion if it has already started, which means it is best applied to brand new vehicles. The major risk to tar-based solutions is that if not applied properly, cracks may develop in the hardened coating over time and trap moisture within itself, leading to rust. Ziebart is one of the largest providers of this method of rust protection and charges approximately $150 per vehicle.

Dripless oil spray

Ziebart also offers something called Penetr-Oil, which is a very dense, wax-like oil spray applied to the entire body of the vehicle. The main draw of this spray is that it is dripless because the substance hardens upon drying, similar to tar-based sprays but it is colourless.

There is a catch though, as this method involves drilling holes into a car's doors, fenders, and other areas to make sure the substance gets applied everywhere. Another known issue is that due to the substance's high viscosity, it cannot get into all the nooks and crannies in the same way more watery oil solutions can. Ziebart charges about $120 for a car and $140 for a van or SUV for this method, and it is recommended that it be reapplied annually.

Drip oil spray

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The rust protection most commonly recommended by experts is a drip oil spray, with Krown and Rust Check being the leading providers. As the name suggests, a slight annoyance is that the sprayed oil drips for about two days after application, potentially leaving stains in your garage, driveway or parking spot.

The key benefit of the drip oil spray is that it is watery enough to reach more parts of your car than the dripless oil spray. Holes still need to be drilled in specific spots on your car to ensure the substance gets applied to all areas and annual reapplication is again recommended. Krown charges about $120 to spray a car and $140 to spray a van or SUV.

To rust-proof or not?

The choice of whether to invest in rust proofing really comes down to how long you plan to keep your vehicle. If you intend to return your car at the end of your lease or you plan to trade-in for a new model before the vehicle's manufacturer rust warranty runs out, then there is absolutely no point in paying for rust-proofing. You can let the next owner worry about it. See this chart for a listing of standard manufacturer rust warranty terms.

For those who plan to drive their cars into the ground, rust-proofing could keep your ride looking good for a bit longer. In any case, you should always consider the cost of rust-proofing versus the price of your vehicle – it would be silly to spend $1,000 to rust-proof a $10,000 car.

Andrew Tai is CEO of Unhaggle, which helps consumers find new car deals by providing access to data on what others paid for the same vehicle, current incentives, invoice prices, and more. Follow him on Twitter @Unhaggle.

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