# Question about surface rust

**URL:** <https://community.cartalk.com/t/question-about-surface-rust/123279>\
**Category:** General Discussion\
**Tags:** bmw, 335\
**Created:** [August 20, 2018, 3:29am UTC](https://community.cartalk.com/t/question-about-surface-rust/123279 "2018-08-20T03:29:30Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![TheMidnightNarwhal](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/themidnightnarwhal/32/5375_2.png) [@TheMidnightNarwhal](https://community.cartalk.com/u/TheMidnightNarwhal)\
**Post date:** [August 20, 2018, 3:29am UTC](https://community.cartalk.com/t/question-about-surface-rust/123279/1 "2018-08-20T03:29:30Z")

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Okay so I jacked my car this week-end and notice some surface rust, especially the diff as the main part of it is in iron or steel.

Now this thought came to mind… when rust starts it usually spreads onto the piece and will rot through right, for example a door panel, it will rot through and make some holes possibly.

Now why is it surface rust doesn’t go through the whole diff if it’s all made of iron? If we go by the logic of rust spreads and gets worse, technically the diff should rust completely inside out after some point?

Is it because the metal is very thick or?

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**Author:** ![oldtimer-11](https://avatars.discourse-cdn.com/v4/letter/o/e79b87/32.png) [@oldtimer-11](https://community.cartalk.com/u/oldtimer-11)\
**Post date:** [August 20, 2018, 4:34am UTC](https://community.cartalk.com/t/question-about-surface-rust/123279/2 "2018-08-20T04:34:03Z")

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Well, since a differential is oil filled it won’t rust inside unless a hole rusts through from the outside and lets the oil out. Eveb stamped steel differential covers are fairly thick and don’t get salt water trapped against them so it takes quite a while. Stamped steel oil pans do rust out around here occasionally but not usually before 15 years. I live near Buffalo NY and we use a lot of salt.

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**Author:** ![COROLLAGUY1](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/corollaguy1/32/7575_2.png) [@COROLLAGUY1](https://community.cartalk.com/u/COROLLAGUY1)\
**Post date:** [August 20, 2018, 9:52am UTC](https://community.cartalk.com/t/question-about-surface-rust/123279/3 "2018-08-20T09:52:35Z")

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You can paint it with black Tremclad rustproofing paint. Did this on my old truck and it lasted until I got wrid of it.

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**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\
**Post date:** [August 20, 2018, 1:32pm UTC](https://community.cartalk.com/t/question-about-surface-rust/123279/4 "2018-08-20T13:32:43Z")

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> [@TheMidnightNarwhal](#):
>
> Is it because the metal is very thick or?

Yes, the thickness has an effect. Cast iron also tends not to get the flaky rust that steel does so it deteriorates much slower. Iron oxide (rust) is a physically larger molecule so when it forms, it breaks off the parent metal. Iron is less dense than steel so provides more “space” for the rust to hang around.

That is usually why manufacturers don’t paint undercar parts made of iron. They just don’t rust away in any “normal” timespan for cars.