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I see a lot of advice online recommending that drivers living in cold climates spray their vehicles' underbodies during the winter to reduce salt buildup and the rust it engenders, but I haven't been able to find any information on how to actually do it. Obviously hooking up a garden hose in the winter is a terrible idea. What sort of device should I be looking to acquire (other than a super soaker) that will hold enough water to spray down a vehicle and will be able to propel it with an appropriate amount of pressure for this task?

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The easiest way to ensure your underbody is kept clean in winter is to find a year round automatic car wash which has under vehicle sprayers.

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    Or a manual car wash and just spray underneath. There is a problem though: you then have to drive home and if OP have Canada-like weather like I have, it is gunk city before you get home
    – Chris
    Nov 24, 2022 at 19:37
  • @Chris - Pretty much doesn't matter. Something you have to put up with all winter long. Montana (where I grew up) never used salt. They put down either sawdust or sand or nothing. Had to learn how to drive on the stuff, but you didn't see a lot of rusted out cars, either. Nov 24, 2022 at 21:09
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    @Pᴀᴜʟsᴛᴇʀ2 Ontario uses salt so heavily, I'm convinced there's an underground salt mafia. Oil spraying is the only way - black cars turn white from the insane amount of salt here Nov 25, 2022 at 19:56
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I understand you are looking for some sort of sprayer to protect your undercoating. You could install a water hose in your garage and find an electric pressure washer to give you more powerful spray, but I'd personally stay away from that option. Spraying your car in your garage (assumed on my part) would increase humidity and probably end up speeding the rusting process elsewhere on your car as it sits in a wet, humid garage. That's not to mention mold that might grow and the large expense of a plumber. Maybe you can find an electric pressure washer with a tank that doesn't require a garden hose.

As far as I see it there are three alternative options for reducing rust progression underneath your car.

  1. Wash your car at a manual or automatic car wash. This will remove salt from important metal components, but they will still remain soaking wet for some time. Water is better than salty water, but water still helps rust spread. If you have scratches and dings that have exposed the metal under your vehicle, you may want to consider another option. I'd consider this a good method for a new car. If you can, do this every time you drive on a wet, salty road.

  2. Get a standard undercoating. Undercoatings cover the important metal components under your vehicle and create a barrier between the moisture and salt. They probably work best if your frame isn't already rusting, since rust usually prevents coatings from sticking to it and it also flakes off and can create openings in the coating. This leads to the weakness of this method: openings can trap water and salt and actually accelerate the rusting of components. This is a good protection method for a new vehicle. It is recommended to do this yearly.

  3. Get an oil-based undercoating like Fluid Film. Fluid film is a thick, oily undercoating that is sprayed onto the underside of your vehicle. It sticks to the vehicle well and prevents moisture and salt from getting to your vehicle's components. Unlike normal undercoatings, this isn't a paint that can trap salt and water behind it. It sticks to the components until it is worn off. This is the method I've used for two years and have been happy with it. It is recommended to get an oil-based undercoating yearly, a month or two before winter. It is also recommended to find a dry dirt road and get as much dry dirt stuck to the coating as you can to keep it in place. You can also use a shop vacuum in the blowing configuration or a leaf blower to get the dirt in place.

I think #3 is the best option because:

  • It works whether your car is new or old and rusty - the rust progression slows considerably when it is not exposed to salt and water at all
  • I've found it to be slightly cheaper than standard undercoatings and frequent car washes
  • Oil is used in your engine to protect it for hundreds of thousands of miles as it scrapes metal against metal repeatedly - coatings and water aren't used for a reason

Its important to note that none of these options are permanent and if you want to keep your frame in safe working condition, regular upkeep is a must. Plan accordingly!

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    Realize oil in the engine doesn't allow metal on metal contact if the lubrication system is working correctly. If there was actual metal on metal contact, an engine wouldn't last but a 1000 miles or so, probably less. Nov 24, 2022 at 19:25
  • @Pᴀᴜʟsᴛᴇʀ2 I can answer that one from watching someone else make exactly that mistake... engine without oil lasted about 6 seconds before it seized! Dec 25, 2022 at 1:40

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