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I received an advice from a friend: put diesel in a bottle with a spray nozzle and spray the car with it. He said that this should form a layer on the car that helps against corrosion.

When I asked how he arrived at such a conclusion, he said that he saw an old corroded car, which was absolutely clean of corrosion where the fuel filler is.

Now, I understand that this is anecdotal evidence at best. However, I was considering if there might be any actual merit in following his advice? He suggested to spray the car once or twice a year (and has been doing so for his own car for a number of years now).

Note: The most related question I found while searching was this. Good answers there, but the question is a bit different.

Note: I asked the same question in Physics StackExchange and was advised to seek better answers from here.

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There is some merit for the suggestion, but with tradeoffs. For the most part, the oil acts as a barrier to oxygen and will prevent a little bit of corrosion. Over time, it'd add up. The tradeoff is, it will collect dirt, which can retain water, which can cause corrosion. If you ever wash your car, all the benefits of the diesel fuel would go away as it gets washed away.

The MUCH better thing to do is to keep your car clean topside and bottom. Then put some type of preservatives on it, like wax or ceramic for the paint. Use undercoating for the bottom. This will not only keep your car looking better, it will help it to keep the rust away.

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    Not to mention the fact that diesel fuel stinks! Wax will give you better protection than diesel with a lot less trouble.
    – jwh20
    Sep 24, 2021 at 10:51
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    yep. this, what the OP's friend suggests may provide some protection, it's quite possibly the worst way to do that Sep 24, 2021 at 13:44
  • Another thing that he said: diesel helps to remove the rust, since it is liquid enough to penetrate the tiny cracks that contain rust. My question is: is this true, and if true, are there better alternatives for acheiving the same effect?
    – Rauni
    Oct 11, 2021 at 8:30
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    Diesel goes away. It's not a rust preventative. It might help for a little while, but not for long. See above for alternatives. Oct 11, 2021 at 12:35
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It will help reduce underside corrosion by protecting it from water. Routine respray ( 3 months ?) would be very good. What I did for chrome bumpers was to dissolve grea se in gasoline and wipe it on with a rag. The gasoline carried the grease into small crevices and it evaporated leaving grease. Bituminous undercoating ( long ago) was found to be bad in Amoco lab tests. Excellent for a while, but then it cracks and salty, wet, dirt is held by the undercoating. Rinsing is worse; anecdotal ,Chicago suburbs. A friend bought a new 1964 Olds and hosed off the bottom regularly during the salty road months (he was a little strange so I am sure he did it). I bought it from him at a good price, it had exceptionally bad rust. It still ran excellently when I drove it to the junk yard before the fenders could fall off (1974).

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