Can rustproofing your undercarriage actually do more harm than good?

Dear Car Talk

Dear Car Talk | Sep 01, 1996

Dear Tom and Ray:

I'm an old geezer who last bought a new car in 1978. Obviously, the
manufacture and the warranties are different on my new 1996 Buick LeSabre.
Living in the rust belt, I can't imagine NOT undercoating the underside of
the car. The salesman says no, that it might void my warranty. And besides,
he says the car is made so well that it doesn't need additional
undercoating. Am I living in the past, or is the Buick Division bamboozling
us? -- Robert

TOM: First of all, Robert, the last time we used the phrase "geezer" in one
of our columns, some nice, older gentleman wrote to us and really let fly.
So we want to make it clear that you're the one who used the term "geezer,"
not us, OK?

RAY: Now, for all the rest of you geezers out there (including my brother,
by the way) here's the answer: The dealer is right.

TOM: I assume you're talking about "rustproofing," Robert, the modern-day
equivalent of undercoating. The problem is that a poor job of rustproofing
(and we've discovered that most rustproofing jobs are poorly done) can
actually encourage rusting! By plugging up drain holes in the
undercarriage, the rustproofing can prevent water from draining, and
actually make the underbody rust faster. So we recommend against it these

RAY: Plus, cars are better made these days. It's not that they won't rust,
it's just that they seem to be taking longer to rust.

TOM: So given the downside of the sloppy rustproofing process, and less of
a necessity for additional rust protection these days, I'd recommend you
skip the rustproofing, Robert. And use your senior-citizen discount at the
car wash a little more often instead.

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