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I was taught that waxing a car helped protect the finish by preventing damage from the elements - water, salt, etc.. The first vehicles I owned (a 1981 Impala and a 1978 Thunderbird) developed larges areas of rust in the body. This could have been from their age (both vehicles were around 8 years old when I drove them) or from my lack of maintenance. I waxed each car only once while I owned it but I did wash them approximately twice a month when the weather was nice.

I've owned more modern vehicles for over 10 years and I've noticed that rust is not as prevalent in these vehicles even though I've done less maintenance on these vehicles with no waxing and less washes. I attribute this, wrongly or rightly, to the advances in automotive finishes.

Is waxing necessary with modern vehicles? I guess I'm looking for justification for my laziness. :)

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tl;dr: Even if you only use spray wax after a wash, it's better than nothing.

It depends on where you live, what kind of driving you do and how you generally maintain your car.

The short answer is that waxing a car puts a sacrificial layer on top of the clear coat which sits on top of the paint (assuming that you have a generally standard paint job). That means that any contaminants (e.g., dust, bird crap, acid rain, etc.) has to get through the wax before it can start bothering your clear coat and then get to your paint.

The rust question is rarely tied directly to waxing, though. For example, when I think of your example cars, I think of rust around the door edges. That was often a case of bad drainage or bad paint at those seams and edges, not a question of wax.

That said, the process of washing and waxing the car generally means that you're spending time going over the finish and looking for problem spots before they get worse. The chances are higher that you'll spot an area that needs touch-up paint before it becomes a big problem if you're already looking at the cat, after all.

Besides, even cheap cars are expensive. It's worth a little time to inspect and maintain your investment. Even if you only use spray wax after a wash, it's better than nothing.

EDIT: responding to the specific question about rust:

Any sacrificial layer acts as a barrier between the ruster (the water) and the rustee (the metal). In addition, it acts as a barrier between the metal and a corrosion-enhancer like road salt. If the salt can get to the metal, it encourages water to concentrate and to corrode the metal. Normally, you have paint and clear coat between the metal and the world.

However, I drive on roads in the real world and, sadly, my paint hasn't remained pristine and perfect.

Even if you have a deep scratch or pit (say from a stone thrown by the truck in front of you) that actually pierces the paint, wax can sit in the scratch and act as a barrier even without paint. This is especially important if you aren't microscopically inspecting your paint all the time (I certainly don't).

Here's what I do when I don't feel like waxing: pour a bit of liquid wax into the bucket before you wash the car. You'll get a layer on the car without a lot of hassle.

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I'll agree that it's certainly a good thing, but as you mention with the door edge rust, there's a lot it doesn't help. Today's cars only very rarely seem to get body panel rust like the old ones did. They're very well protected from the factory. New cars really only get rust if/where water gets trapped (like those door edges). Floor surfaces are also vulnerable since old door seals will leak and may go unnoticed (bottom layer of carpet is wet and you never see it). Strut towers and parts of the frame also tend to go earlier than body panels due to water getting trapped. –  Brian Knoblauch Jul 4 '11 at 16:38
    
I believe your asserting that waxing does not affect the rate of rusting for a vehicle, correct? So if rust is the only concern I have in regard to my car's appearance then I do not need to wax. –  Muro Jul 4 '11 at 17:04
    
@Muro, edit added to the answer. –  Bob Cross Jul 4 '11 at 21:38
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@Brian, as you point out, wax isn't magic. However, in all of your examples, wax wouldn't make the situation worse. –  Bob Cross Jul 4 '11 at 21:41
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@Muro Realistically, the wax probably won't have any impact on the rusting. However, it will protect the paint and keep it looking nicer a lot longer. Can save you a paint job down the road... :-) –  Brian Knoblauch Jul 5 '11 at 13:31
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You can't wax under your car and that's where the rust is/starts. Today cars are much better protected with galvanised body panels and therefore wax on the outside of the car is only cosmetic. Bought a Triumph Vitesse (Brand New} in 1969 and it already had rust on the body seam. We wouldn't stand for it today.

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