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There’s been a lot of past discussions concerning about undercoating and I get an impression that I should be glad the dealership saved me the time for making the pitch.

Recently, my wife and I purchased a brand new 2010 Subaru Forester. When we showed the Forester to my in-laws, my father-in-law said we should get undercoating for the car to protect it from rust and what not. We’re moving to Colorado and we’ll be driving from DC to Denver after Christmas. We’ll be spending a lot of time in the mountains to visit my sister-in-law’s family and to explore. My father-in-law said it would be a wise investment to get the undercoating. My father-in-law is a pretty smart guy, but this is one time when I think he may be wrong. I’m under the impression that new cars nowadays do not require undercoating and that it might do more damage than good in the end.

Does my father-in-law have a point about undercoating?

No, you’re correct. Don’t get the undercoating. At best you will accomplish nothing, at worst you will trap salt and other crud underneath the coating where it will happily eat away at your car for years without ever being washed off.

Cars are already rust-inhibited at the factory. Further applications are unnecessary and just a drain on your wallet.

People confuse undercoating with rust-proofing.

Undercoating is applied to the underside of the vehicle to reduce road noise. Rust-proofing is applied to inner panels of the body. This is done by drilling small holes in areas of the body so a wand can be inserted and then the rust-proofing is sprayed onto the inner body panels.

Applying an undercoating to the underside of a vehicle where those areas aren’t absolutely clean prevents the undercoating from adhering completely. These areas then provide a place for moisture to collect resulting in rust.


Your father-in-law is right, but he is a few decades back. Modern not only don’t need after market undercoats, but they do better without them. After market applications often defeat the built in anti rust designs, like drain holes and end up causing rust. Be nice and if the subject comes up again, say you checked and found out that it was treated at the factory and they may not honor the warranty if you apply something to what is there. (Which is likely right.)

Thanks for the info. If he brings it up again, I’ll know what to say.