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I drove a '97 Subaru Legacy for many years and that thing got over 280,000 miles and was still running smooth before rust finally killed it. The frame started rusting away; there was a hole straight through to the trunk interior from the outside. It was quoted at 10x the cost of the car to repair; the rust was very extensive, like metastasized cancer.

Because I have a hard time moving on, I just bought another '97 Subaru Legacy.

How can I keep the same thing from gradually happening again?

I should add that I live in New England, where snow and salt on the roads are a yearly norm. Does the salt have anything to do with it? I've been told it does.

  • 1
    waxoyl, dinitrol ... Go on any UK based Land Rover Defender forum and see the discussions there. Two reasons: UK salt their roads in the winter and the Defender is a known rust magnet. – Lamar Latrell May 19 '17 at 22:22
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    I'm in California so our cars never rust (i've never even seen rust on a car and we still have people driving around classics from the 40's). However I've always wondered if you could just install a sprinkler head in your driveway to wash the salt off your car when you get home. – user959690 May 30 '18 at 16:47
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Rust is a chemical reaction that occurs when metal contacts air and water. The type of reaction is electrochemical, meaning electrons are being exchanged, and salty water being very conductive just speeds things way up. Knowing this, you have options:

  1. Don't let the car get wet. Don't drive in wet weather and don't park the car outside. Ever.
  2. The more access to metal the air and water have, the faster the reaction will occur. Cover up your metal (this is the chief reason why cars get painted at the factory. It ain't for looks, it's fundamental to rust prevention.) Undercarriage coatings help, but be advised that rock chips will defeat the purpose of this. Regularly repaint the underside of the car, the thicker the material you apply (or more coats), the better. Some primers have rust inhibitors in them, use one of those.
  3. Electrochemical reactions are one of "preference." Water and air will react with the most reactive metal they can find. There are metals that are more reactive than steel, place a substantial quantity of one of those in direct electrical contact with the steel of the undercarriage. This is called a sacrificial anode. I'm not aware of a commercial product available for cars that does this, but this technique is used extensively with sea-faring ships with steel hulls as well as gas stations with underground steel tanks (they tend to use plastic these days). Common metals used as a sacrificial anode are zinc and magnesium. Understand that this metal will rust (that's the point), but as long as there's some of it left making electrical contact with the steel of your car, your steel won't rust. You would be sacrificing the zinc or magnesium (whence the name) to save your steel, and as such would need to replace the zinc or magnesium as it runs out.
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    @tlhlngan do them electronic rust inhibitors work? – Chris May 19 '17 at 22:50
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    I'd love to see a sacrificial anode for a car application. Kind of odd it isn't already a thing, or at least a thing I've heard of. – atraudes May 20 '17 at 0:22
  • @atraudes Cars a perceived to be a disposable commodity. Most people buy new cars and trade them in long before rust appears. – tlhIngan May 20 '17 at 2:54
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    @Chris I've never tried them, but the principle is sound. It's called cathodic protection, and works by providing free electrons in the form of an electric current. The air and water are trying to steal electrons from the easiest source (steel, in our case), this device provides electrons more easily to the air and water, therefore, in theory, no rust. This could run your battery down though. – tlhIngan May 20 '17 at 3:00
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    @SteveRacer We were talking about cathodic protection running your battery down. Sacrificial anode doesn't. – tlhIngan May 20 '17 at 7:05
2

Rust as any chemical reaction will have higher rates at elevated temperatures. It is often falsely believed that your car rusts during the winter time when there is salt on the roads. Actually, your car rusts after the winter, when the car's frame and undercarriage are covered with salt but when the temperatures are elevated.

So, to prevent your car from rusting again, I would advise you to do several things:

  • Purchase your cars as new. Old cars have already rust and there's nothing you can do to prevent it from expanding.
  • Get a rust protection job for your car. Immediately! As the first thing you do with your car! Expect to pay around 1000 USD, mostly for labor. A good job takes about three days. In areas where salt is often used on roads, rust protection shops are prevalent (at least in Finland I know there are many rust protection shops near the capital area).
  • Take your car for a rust protection shop for inspection once per three years. Let them do whatever fixes they think are necessary.
  • Wash your car very thoroughly immediately after winter, when salt is no longer used on the roads but when temperatures are not yet elevated like during the summer. This prevents the salt from causing the chemical reaction that turns your car to rust.

Now, you said you bought a 20 years old car. Car of that age is expected to have lots of rust already. Did you inspect it thoroughly for rust with a flashlight under the car? If not, I would say the expected lifetime of your car is about 2 or 3 years based on the information that I have. A 20 years old car is going to have other problems than rust as well, e.g. you can expect pretty much everything made from rubber (oil seals in the engine and in the power steering rack, fuel lines) to fail.

If you really, really want to buy second hand cars, do buy from a place a bit further away where salt is not used on the roads and thus rust is not an issue. Even then, you should verify the car thoroughly for rust. Then let a reputable shop do the rust protection job. Note that for old cars, you may need to pay extra in addition to the usual price of about 1000 USD. This is because they may need to do something for the rust that is already present.

Is the meticulous care against rust worth it? I would say it is, especially if you prefer larger and more expensive cars. My car cost about 50 000 USD. I expect to get about 20 years of lifetime from it, thus costing 2500 USD per year. A 300 USD rust check once per 3 years adds 100 USD per year. A 1000 USD rust protection job adds to the price resulting in 51 000 USD, but if my car now lasts for 25 years, it's 2040 USD per year plus 100 USD per year or 2140 USD per year. Somewhat cheaper that way: you save 360 USD per year.

Now, consider a small car that costs 17 000 USD and requires a 700 USD rust job (cheaper due to needing less chemicals) and 300 USD rust check once per 3 years. It's 850 USD per year if the car lasts 20 years without rust protection, and 808 USD per year if the car lasts 25 years with rust protection and with regular inspections. Now you save only 42 USD per year. Is it worth it? Probably not, considering that rust is not the only enemy of old cars. Your car may fail before rust starts to be an issue.

These are prices converted from Finnish prices using the conversion 1 EUR = 1 USD. In areas where taxation is lower, prices may be lower as well.

  • I used to work in the auto sales industry. We had an aftermarket team do the undercoating, paint sealant, & Scotchguard (for the fabric). It was sold as a package. We'd normally charge the customer between $400-600 for the package. Thing was, it only cost us $60. If you know someone in the industry, you can get it done for really cheap. If not, I highly recommend you do this yourself, as cans of the undercoating are pretty cheap in comparison to what it will cost you going through the dealership. – Pᴀᴜʟsᴛᴇʀ2 May 20 '17 at 17:25
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In the winter time you have to wash your car!! Because salt turns your car into a mountain of rust. You could buy a car wax and grease it on your car that avoid rust as far as I now. Avoid roads with "tons" of salt, park your car in a garage or on a dry place!

  • I'm not sure how waxing your car is going to prevent the frame or under pinnings from rusting? – Pᴀᴜʟsᴛᴇʀ2 May 20 '17 at 17:28

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