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Besides visual examination, is there some way to find out reliably how much a given car is affected by rust? I mean, which parts (possibly covered by paint) are rusting in the inside, and to what extension?

I'm aware that there are instruments for measuring paint thickness, but as far as I understand they do not tell if the part below the paint is rusty or not. I've tried to do some research, and it really seems to me that the best one can do is to visually examine the car (also from below, by lifting it), and from the rust spots, try to extrapolate how much the car might be rusty on the inside.

Is this really the best one can do, or are there any other cheap and non-destructive ways which give a more precise result than visual examination? (I.e. some kind of instrumented measuring.)

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    Theoretically, if you knew the dry weight of the vehicle, you could weigh it. Rust (iron(II) oxide) is heavier than iron, so a rusted car will weigh more, if none of the rust has flaked off. – Hari Ganti Apr 11 '17 at 18:29
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It may sound old fashioned or not professional but you really cannot beat a hammer. A ball pein hammer is best as it will not split the paint work.

Tap the suspect areas and compare the sound and feel to a known good area with the same thickness of metal. A good piece of metal will produce a sharp tapping sound and will have a hard feeling, a bad piece will have a dull sound and will feel different almost like it is more cushioned.

If there is a really thick layer of underseal or stone chip where there maybe no difference in sound between a good and bad piece of metal you can push onto the metal with an object such as a screw driver. If you go through it's obviously rusted out, it doesn't matter if you do as it would have required fixing anyway.

You could also use a borescope (a small camera on the end of a flexible cable) you can push this through existing holes in the inner sills, cross members etc to few the inside to see if they are rusting from the inside out.

Don't be scared of surface rust either from the inside or outside as this can either be cleaned and painted or treated with a wax-oil spray.

  • Thanks, these are good ideas indeed, at least for my own car. However, I ask myself, if they can be applied for a used car too. If I were buying your car, which one of these (ball pein hammer/screw driver/borescope) would you be fine with me trying? – Attilio Dec 2 '18 at 10:46

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