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You & Your Car

Should I oil spray under my car? Add to ...

Do you have any opinions on oil spraying under the vehicle as a rust protection? – Peter Coffin

I do. Spraying oil on the underside of a vehicle is a messy and temporary preventative measure with major drawbacks.

First things first, it is cheap, because used motor oil is otherwise useless and the only cost should be the application.

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It is messy because that used (or even new) oil is not meant as a surface protectant and will drip off until what remains has dried out. It is temporary because, after it has dried out, most of the protection disappears.

But there are two major drawbacks:

1. Oil is not formulated to stick around, to remain in place and provide protection in ambient conditions.

2. The underside of the vehicle is not where rust problems lie.

The underside of a vehicle may appear rusty, for example, have a thin coat of rust on some exposed and unprotected surfaces. But it is exposed to the drying air as the vehicle moves down the road. And, if there is even a tiny fluid leak from the engine or transmission, you get a free coat of oil sprayed underneath the vehicle.

Because of mandatory rust perforation warranties, manufacturers go to great lengths to ensure new vehicles remain rust-free. Most critical metal parts of a motor vehicle, including body and structural steel panels and pieces, are galvanized, covered with a protective coating. Unless and until this coating is compromised, there is little opportunity for rust to start.

Where it does become a problem is in hidden cracks and crevices not reached through a simple underbody spray. Fenders and doors are pre-drilled with tiny holes to allow moisture that may accumulate inside to drain off. If those holes become blocked and the moisture accumulates, rust may get an opportunity to get started. However, this would not be addressed by spraying oil on the undercarriage.

Where metal components are joined by welds, bolts or other bonding methods, they may work against each other wearing off the protective coating as the vehicle flexes. This is extremely rare in a modern vehicle whose structure has been engineered to provide protection in a crash.

But if it does happen, it will likely occur in a place not reached by spraying oil under the vehicle.

In short, spraying oil under a vehicle was a good idea and had its benefits in the “old days” before galvanized metals and other preventative measures were introduced at the manufacturing level.

You will hear claims like, “I’ve had all my cars sprayed with oil for 25 years and have never had a problem with rust.” My answer is that the same person probably would not have had a problem if they had not sprayed.

Send your automotive maintenance and repair questions to GlobeDrive@globeandmail.com

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