Can I spray my undercarriage with motor oil to prevent rust damage from salty Vermont roads?

Dear Car Talk

Dear Car Talk | Jun 01, 2004

Dear Tom and Ray:

I live up in Vermont, and I just bought a new Toyota Tacoma. Many of the people in my town tell me that they protect their vehicles from winter salt damage by having their undercarriages sprayed with used motor oil. There is a guy who has a business doing just that. Apparently, it's legal up here, and it's not terribly expensive. Do you think it's an effective measure to take, and does it do any harm to the vehicle? -- Roger

TOM: As far as we can tell, Roger, this is an old-fashioned remedy that's been passed down in rusty old places in the great frozen North. And like a lot of old folk remedies, it has its roots in truth.

RAY: But it's been surpassed by more-effective, environmentally friendly solutions.

TOM: In the old days, before metal body parts were "dipped" and rust-proofed at the factory, cars and trucks in the great frozen North used to rust out in no time. It wasn't unusual to see a car with its body rusted through after only five or six years.

RAY: So, soaking the whole underside of the car with used motor oil seemed like a good idea. It does inhibit the rusting process, at least for six months or so. And it was in plentiful supply. Heck, you were going to throw it away anyway, right?

TOM: And did it hurt the car? Well, probably not. You wanted to avoid rubber bushings and stuff that could be degraded by oil. But as long as you closed the doors first, you were usually OK.

RAY: The problems are that, aside from being less necessary nowadays, it's a terrible thing to do to the environment, and you have to do it every year. As you can imagine, in addition to the flood of oil at the application site (unless the guy uses an approved collection system), you drive away dripping the stuff all over the state.

TOM: Given the strict regulations shops face now in disposing of used oil safely, I can't imagine that anyone in charge of environmental regulation or drinking water is going to look kindly on this process. So I'd definitely double-check (or triple-check) the legality of doing this.

RAY: Our recommendation would be to skip the used-motor-oil shower, Roger. If you feel you need more rust protection than the factory provides, I'd ask a mechanic about applying a rust-proofing product designed for that purpose. They cure within hours, provide a much cleaner solution and can be checked and touched up annually, as necessary, without environmental damage.

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