(Yep, photographic film, the real deal.)

  2015 November 23rd     Tech   Metal & Shop


Steel and iron get rusty. 

Rust can eat through a car frame.  It can devour the sheet metal.

Rust can turn a nice vehicle into a heap of crumbly flakes.  The names "Mercedes" and "BMW" have no special status in the eyes of rust;  they're just another couple of tasty snacks for it.

Let's look at some ways to deal with car rust.   In this article, we'll focus on the relatively easy, low-cost methods of rust protection.

Much of this will also apply to dealing with rust elsewhere, such as tools.


A Quick Note

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In This Article

Before You Do Anything Else

What Is Rust?

Surface Prep

Oil-Based Protectants


Before You Do Anything Else

Mechanical Safety

Never get under a car that's supported only by a jack.   You could get crushed to death.

Use a set of good jack stands.  Cheap ones fail, often.  People get killed this way.   This is one piece of equipment where you should never skimp.  And also, make sure you're always on solid pavement.  Jacks can sink into the ground when there's a car on them.



Many "green" chemicals are still dangerous if you inhale the mists.  Doesn't matter if it's a hand sprayer, a pump sprayer, or what;  get a cartridge respirator.  This one from MSA works well.   Make sure the cartridges are P100 and they're attached properly.

Be especially careful with mists that contain acids, oils, or surfactants.  That's most automotive products!  Just wear a respirator.

Also, most anything oily on your hands will eventually get through your skin.   No problem:  that's what gloves are for.  Get a pack of disposable nitrile gloves


Be careful around oxygen sensors, CV joint boots, rubber hoses, and anything that's not metal.  I haven't melted anything yet, but you might.  And I'm not responsible if you do.   Spray carefully.

Also, use cardboard or something to catch the drips of whatever you spray.  And make sure the car is not dripping anymore when you go to your friend's house and park on his nice new paved driveway.   

What Is Rust?

Some people think rust is a pure substance.  Nope.   Rust does not have a consistent formula, contrary to what you may read elsewhere.  I know a thing or two about this subject.

When you see "Fe2O3", that's a gross oversimplification.  It's not the exact formula of rust.  

Rust is a mixture of iron oxides, hydroxides, oxy-hydroxides, et cetera.  There can be varying amounts of water.    There are a couple of iron compounds that are pretty consistently found in rust, but there can be a bunch of variants, depending on the water content. 

"What is rust?"  The technically-correct answer is too darned complex for a short article.  

Which, I guess, makes it sort of OK that most people just say "Fe2O3".   And if you bake rust long enough, you do in fact get Fe2O3.  Mostly.

One thing is certain.  Rust occupies more volume than the original iron.   That means the rust will expand, flake away from the iron, and let more water in there.   It won't stop until there's no more metal left.

Don't let your car get to this point. 
Even plain ol' WD-40, which is a bit thin for undercoating,
would probably have been better than nothing.

Very often, rust can have micro-droplets of water in its matrix.  In fact, most rust probably has this, unless it's continually baked (like the rust you find on an exhaust system.)

As you might guess, trapped water could promote further rust. 

Surface Prep

Beforehand, you should have hosed down the underside of the car with clean water, to wash away any salt that might be there. 

Then, let 'er set for a couple of days.  Pick a time when there's no rain in the forecast.

The metal has to be very dry before you spray on any of the protectants.

Use a putty knife or a wire brush to remove any loose rust.    (Every toolbox should have a putty knife.)  Wear eye protection from falling rust flakes!

Oil-Based Protectants

If oil floats on water, then how can oil protect metal? 

The answer involves a lot of tedious physical chemistry.  

Basically, if you oil a piece of dry metal, it creates a surface that has a low surface energy with respect to water.  That means the water will bead up. 

There are different types of corrosion inhibitor chemicals.  Some are too nasty to use in consumer products (N2H4 being a prime example).  Others don't mix well with oil.  Therefore, most of your spray-on rust protectants work by surface adsorption.  That's "adsorption" with a "d".  

Now, some of them may have top-secret corrosion inhibitors added, too.  However, adsorption is still really important.

Most oils have a lower density than water.  As you know, that means oil will float on water.  That's OK, though.  If your oil formula can achieve good enough adsorption to the surface, it can still block water out... up to a point.   Pure mineral oil is a crummy rust protectant, because it does not adsorb well or create a good film.   (I have a corrosion test running right now;  pure paraffinic mineral oil is looking worse than the control.)

Very thin coatings can get pushed away.   Spray enough water for long enough, and that happens.   That's why thicker and greasier undercoats are usually better.   That, and they also seal out oxygen better.  Remember, to get rust, you need more than just water... you need oxygen to be present, also.

A heavy coating of axle grease might work, but do you really want to sit there with a little brush and apply that to every square inch of the undercarriage and sheet metal?

Let's talk about some products that can be sprayed onto the metal.

WD-40 works well in the short-term.  If you were going off-roading and just needed some metal protectant for a day or three, it would work OK. 

However, I wouldn't rely on one application to get through a whole winter. 

WD-40 is just fine for spraying on stuff that will be kept in a toolbox.  WD-40 is also great if you just want to spray down some tools for a day of working outside when it's humid, foggy, or drizzling.   It smells good, too, though sometimes I wish they'd "turn down the volume" on the fragrance maybe 20%. 

Undercoating a car is a special task.  There's continually going to be water spraying on it from the road.  And that water may have salt in it.   In the world of anti-corrosion sprays, there isn't much that can keep functioning under those conditions, actually.

Instead of regular WD-40, you'd get better results here with WD-40 Specialist Long-Term Corrosion Inhibitor (LTCI).  This leaves a heavier residue than regular WD-40, so it would be a lot better for car rustproofing. 

WD-40 LTCI is one of the better protectants against salt spray, but the spray cans are small.   Most other anti-corrosion sprays are available in 11-oz cans, or somewhere thereabouts.   LTCI is sold in 6.5 ounce cans.

For a car, figure on needing at least thirty or forty ounces of product.

That 6.5 ounce can makes for a nice stocking-stuffer size, though.

PB Blaster inhibits rust about as well as WD-40 not quite as well as WD-40.    (Updated this after staging a mini-shootout between these products. Results were surprising.)   More important, it's thin like WD-40, and much of it evaporates.  So, it's great for indoor stuff, and it's one of my favorite automotive chemicals... but I wouldn't use it for a winter-long vehicle undercoat.

For that, consider the more specialized PB Blaster Corrosion Stop

PB Blaster + Gear Oil

For stuff that lives outside constantly, straight PB Blaster leaves a coating that's too thin, and it dries too much.

I tried a mixture of two different liquids in the hope of a better anti-rust coating...

Formula:  10 to 25% gear oil, 75 to 90% Blaster (get the gallon size). 

Apply the mixture with a good spray-tip oiler like this one.  

PB Blaster is thin, evaporates slowly, and has a very low surface tension.  That gives the thinned-out gear oil too much time to seek gravity. 

That means the surfaces farthest from the earth will not have as thick a film, unless they're perfectly horizontal.   

I tested this fine petroleum-smelling mixture on a short piece of chain.  The chain hung in a U-shape outdoors, and water was still beading up on it weeks later.  (Yeah, buddy!!)  Only problem is, by then you could see that most of the gear oil had crept toward the bottom of the "U".  The upper portions had only a trace.

If you don't already have both these ingredients sitting in your garage, I would suggest starting with a purpose-built product... such as, maybe the Corrosion Stop from Blaster Corp.   And save the regular PB Blaster for bustin' loose stuck bolts and that sort of thing.  (It is also a rust protectant... just not a long-term outdoor rust protectant.)

LPS-3 is among the best for long-term outdoor use.  It's used a lot on military aircraft, which speaks of its effectiveness.  LPS-3 contains no silicones and no chlorinated solvents.   I'm sure the military is probably wise to the whole "stress corrosion cracking" thing that can happen with chlorine-based products.  (It appears they knew about this in the Sixties, based on the mil specs for corrosion inhibitors.)

Along with Fluid Film, LPS-3 has the right chemical profile for a corrosion inhibitor. As far as I know, there aren't any questionable ingredients where you'd have to worry about "just the right conditions" happening and leading to grain-boundary stress cracking, accelerated rusting, or anything like that.

These products also have the right consistency to use on outdoor stuff.  In other words, metal that's going to get a lot of rain, splashes, and road-salt spray.


Fluid Film is what I would consider the gold standard of corrosion inhibitors.

(120studio.com     /       Disclaimer)

Fluid Film contains lanolin, which is very tenacious on metal surfaces.  Of course, it's blended in a formula that most likely optimizes this property.

There's a lot of variation in the "backyard corrosion tests" out there, and sometimes it's tough to make sense of all the different results.  That said, Fluid Film is one of the most consistent winners.  Once you eliminate the products that have silicones, PTFE, and chlorinated esters... that gets rid of most of Fluid Film's would-be competitors.   (Very few of them could approach Fluid Film anyway.)

I've been looking for a really good rust protectant that didn't have any of those three things in it: chlorine, PTFE, or silicones.   

Fluid Film is partly petroleum based (a good thing for rust protection), and it uses lanolin (also good).   It does not contain any solvents.

It's not that solvents are bad in and of themselves, but think of this.  No solvents means you are getting a higher percentage of useful components, instead of ones that evaporate away.  So, it's a better deal on that basis.

Like LPS-3, a coat of Fluid Film is more like grease than like the thin oil you might have been expecting.  I say that because you might be conditioned from years of using everyone's favorite "water displacer".

You'll probably want to buy more than just one can of Fluid Film.  It's very versatile.

Get Fluid Film in multi-packs of aerosol cans, or in 1-gallon pails for undercoating.  If you get the gallon container, you'll need your own sprayer if you want to do a car undercoat.  (You could brush it on, but reach is limited.) Many people use air-compressor sprayers for this.

The one thing I don't like about air-compressor sprayers is that water vapor also gets liquefied, so you end up spraying fine water droplets mixed in with the spray.  You can avoid this by having good water traps in your compressor line, but many people don't do this. 

Easiest solution?  Just get the aerosol cans.   If I had to choose one rustproofing product for cars, and it had to be ready-made, it would probably be Fluid Film.



This has been a look at how to keep metal from rusting.  In this article we've looked at the easiest method, which is a liquid-based product that you just spray onto the metal. 

Bottom Line:  get yourself some LPS-3 or Fluid Film

The fairly new WD-40 LTCI is also a possible contender, though the cans are rather small.
Once again... the metal should be dry and salt-free before spraying any of these.  You might as well clear off the rust flakes, too. 

In a future article we may look at other methods, such as rust converters and paints.  The sprays I've talked about here could be all you need, though.  Just re-apply the product every season.

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