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2012 SH Manual Diesel
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788 Posts
Salt does not cause rust;
Not arguing, offending, or point scoring; just broadening the perspective! And I agree with your rust prevention experience and advice, so no contest there.

I know that what you are saying can be technically correct in principle, especially for the defined basic chemical reaction. It is also how Redox Reactions are taught in Chemistry (the minimum components required for a straight forward reaction). But where chemistry teachers stop; is where "really!?, how did that happen", begins. It's the sources of the Proton donors (in the textbook, they conveniently just give you these 'Free' Hydrogens, as if they were just standing idly 'waiting for a mate'.); changes in electrical potential (in electrically conductive material or solutes. Galvanic/Daniell cell); intermediatory reactions and by-products; or presence of catalysts; where the salts fit right into the equation(s).

However, in 'bucket Chemistry' (how poop typically happens in real life), chemical reactions can be complex, or very round-about (not straight foward), and can take many routes or intermediatary steps. It's not necessarily obvious how salt cause corrosion; and in some aspects, it can be quite cryptic. You have to think laterally or tangibly, then you see the associations.

To make sense of what I'm saying, you have to consider that; if the salt was not present, then the Redox Reaction would not occur (or not at an accelerated, detrimental rate). And that the Anode and Cathode could be different metals, on different panels or components, or a long way appart. Some example(s), are:
  • Bubbling paint, for no apparent reason (no scratch; no hole in panel; no rust any where near the area; surface coating completely intact, and bound all the way around the bubble); but yet corrosion is occuring under the paint;
  • Stray current corrosion in cooling systems;
  • Rusting bolts, but where no other exposed metal component is corroding.

The following are some of the principles/concepts of how salt provides the required elements of corrosion to start:
  • Daniell Cell: salt bridge;
  • Salt: weak acid + weak base (include carbonates, nitrates, sulphates, and hydroxides);
  • Carbonates, Nitrates, Sulphates: e.g. Carbonic, Nitric acid (acids are corrosive on metals), they also contain Oxygen;
  • Hydroxides: Contain Oxygen and Hydrogen ;
  • Hydrogen: required for reduction of Oxygen at Cathode;
  • Salt: unless anhydrous (Lab supplied), will naturally contain moisture.

And due to electrolysis; rust begets rust!
 

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2015 Forester 2.5i Limited CVT
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If you want to get a liquid like Evaporust on undercarriage parts, soak paper towel in EvapoRust and try to lay them on rusted parts.
Tin foil will help keep them on. First wire brush the rust. Forget the exhaust for now.
 

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2018 Forester
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164 Posts
I applied fluid film when my car was new. I believe that is the difference with it working.
Rust is rust, but if you apply before rust then your good.
After rust you need something else. I used to oil my old trucks but your not supposed to do that now.
 

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2010 2.5X Manual
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311 Posts
I grew up walking barefoot on oiled dirt roads.

I will probably put some type of oil prep on mine, actually I wish I had thick silicone electrical grease in a spray can.
I can oil it over cardboard in the garage then let it drip and burn the cardboard.
 

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189 Posts
Look into Woolwax, or NHOil undercoating. Both are non-toxic non-drying rust prevention spray. It does not dry so it wont chip/scratch and has been used for years esp on the east coast and Canada.

The old days people sprayed various autmotive oils because it stopped current rust and prevented new rust but its very toxic to the environment and eats rubbers and poly material. Imagine the same process but with a non-harmful and non drip oil type product.

It will not just trap the rust like spray painting over it. It will stay wet and absorb into the metal pores and stop rust from progressing and keeping oxygen/moisture from reaching it or other metal. The underciarriage just has a wet look and is really the only way to stop rust on a new OR USED vehicle. It wont hurt the bushing or body mounts or eat at the soft coating from the factory. I used fluid film for 6 years and my cars and trucks are 99% rust free. I switched to woolwax in 2018 because it has almost zero small and is WAY THICKER... fluid film would rub off from snow/dirt little easier than I liked but all the enclosed spaces sprayed with 460 degree wands stay wet and rust free almost indefinitely.

Any "mechanic" who thinks it will make things worse is uneducated with woolwax, fluid film or NHOil. They do not dry and harden like paints, and they are not a rubberized coating like bedliner. You can research it and see the reviews on youtube and their websites have undercoater locators.... worth every penny to extend the life of your vehicle. My f250 diesel, my 09 forester, and my work trucks are sprayed every 2 yrs and I can get any nut/bolt off without using a torch or breaker bar... Yes I have an oily residue on the undercarriage but no more rusted out panels or breaking bolts. Its a small price to pay for rust free ownership. Its been used for many many years its just not a huge industry.
 

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2010 2.5X Manual
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311 Posts
That's what my intuition was, anything that doesn't form a skin that moisture could get trapped behind wouldn't make it worse.
 

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I tested out the aersol cans on my personal truck after one year no new rust and the oil film was still on the current rust spots. I then upgraded to the undercoater who uses a high pressure airless system from Canada and they throw it up on a 4 post lift and pull the plugs/drains from the body, doors, any drain points in hood and then tailgate, trunk/hatchback. Then spray inside any enclosed areas. I can take a pic of the wifes forester where you can set wetness that creeped up from the bottom underbody plugs to the exterior. Its below the plastic liner and just looks like 4 1/8 moon shadows from the bottom but it shows how much it creeps.

It has not been an issue for brake changes, oil, transmission, coolant fluid or dif changes either. Again just a pair of disposable gloves that one would wear for an oil change anyways lol. Contact an undercoater and you'll see how they roll. Some even use a modified wedge trailer for mobile services
 

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That was the woolwax?
I used fluid film first but its quite a bit stronger small and not as thick. Woolwax uses lanolin as the same base ingredient but formulated with a much thicker material and way less smell. I still have some fluidfilm cans for stuff like bike chains, snowblower before storage and other random stuff. It pretty much replaced wd40 because woolwax and fluidfilm have no solvents so it doesn't dry or evaporate. WD40 is still good but it dries out eventually so it shouldn't be used on things like hinges and chains for regular lubrication its more of a cleaner then you apply a longer lasting lubriant etc.
 

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2020 Outback
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Look into Woolwax, or NHOil undercoating. Both are non-toxic non-drying rust prevention spray. It does not dry so it wont chip/scratch and has been used for years esp on the east coast and Canada.

The old days people sprayed various autmotive oils because it stopped current rust and prevented new rust but its very toxic to the environment and eats rubbers and poly material. Imagine the same process but with a non-harmful and non drip oil type product.

It will not just trap the rust like spray painting over it. It will stay wet and absorb into the metal pores and stop rust from progressing and keeping oxygen/moisture from reaching it or other metal. The underciarriage just has a wet look and is really the only way to stop rust on a new OR USED vehicle. It wont hurt the bushing or body mounts or eat at the soft coating from the factory. I used fluid film for 6 years and my cars and trucks are 99% rust free. I switched to woolwax in 2018 because it has almost zero small and is WAY THICKER... fluid film would rub off from snow/dirt little easier than I liked but all the enclosed spaces sprayed with 460 degree wands stay wet and rust free almost indefinitely.

Any "mechanic" who thinks it will make things worse is uneducated with woolwax, fluid film or NHOil. They do not dry and harden like paints, and they are not a rubberized coating like bedliner. You can research it and see the reviews on youtube and their websites have undercoater locators.... worth every penny to extend the life of your vehicle. My f250 diesel, my 09 forester, and my work trucks are sprayed every 2 yrs and I can get any nut/bolt off without using a torch or breaker bar... Yes I have an oily residue on the undercarriage but no more rusted out panels or breaking bolts. Its a small price to pay for rust free ownership. Its been used for many many years its just not a huge industry.
I concur. I started using Fluid Film a year ago. The undercarriage My 2020 Outback (after 18 months in the midwest) still looks like brand new. My wife's 2019 Forester is almost 3 years old, and it's undercarriage looks pretty good. I also have a 2008 Civic that was showing signs of rust. I wire brushed the loose scale, then treated it with Loctite Extend rust nuetralizer to convert the rust to an inert coating. I then sprayed the treated parts with Fluid Film to slow down any new rust. This seems to have worked.
Regarding the cast iron parts: when rust forms, it does not seem to weaken the parts. The iron rust seems to stop the erosion so it remains just surface rust. Steel, on the other hand, seems to rust in layers. The first layer flakes off and the layer underneath continues to rust. So, you need to stop that rust. Try the Extend on that, then coat with Fluid Film.
The Fluid Film IS water soluble, so respraying every year is good idea. I always keep a can for spot treating. It has a curious odor that takes a week or two to dissipate, but as others have posted, it lubricates threads, so it will prevent nuts from seizing.
 

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2015 Forester 2.5i Limited CVT
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I always worry that rust is getting trapped under all that oil film mess. I work under my car a bit, so having all that oil on everything makes it a no go for me.

I’m in the South so rust is not a big issue. I’ve only use Rust converter on some spots, back of muffler for instance. BBQ black. Or just replace or remove/paint.

Brake calipers are on the hit list but they look good so far.
 

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2018 X3 M40i / 2016 X3 xDrive35i
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1,632 Posts
I always worry that rust is getting trapped under all that oil film mess. I work under my car a bit, so having all that oil on everything makes it a no go for me.

I’m in the South so rust is not a big issue. I’ve only use Rust converter on some spots, back of muffler for instance. BBQ black. Or just replace or remove/paint.

Brake calipers are on the hit list but they look good so far.
Try spraying oil on water, they don't mix... then take a wet surface and spray it with oil...the surface becomes oily and the water balls up on top. Fluid film pushes the water and then it doesn't let it back in. It's not the same oil as a petroleum based oil....it's derived from sheep and the product will wash off and the metal look clean if not a bit aged much like a broken in cast iron skillet. I've also found this metal that has soaked in some of the product resists rust more than entirely untreated metal as well. Project Farm has tested it a lot as well.

 

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2015 ; 2.5i ; 15,000 Miles (I drive a lot less than I used to !!!!)
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144 Posts
This cannot be good... I'm in the midwest, and my 2017 forester I bought last summer was parked outside for the past winter. I don't know how much corrosion there was before I owned it because I either never thought to check or didn't notice if I did... What should I do now? These photos were taken tonight, and no, I cannot sleep.
View attachment 554034 View attachment 554035 View attachment 554036 View attachment 554037
You would be better served if you were concerned about the under-carriage metal that is specified by Gauge-Number, than you are about heavier parts that are manufactured from Dimensioned-Stock. Specifically, watch or take action on the under-side areas of the Floor-Pans, etc. The thinner the ferric part the more you should be concerned about long term thinning by rusting .....
 

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I understand. My concern is water trapped or rust trapped under any material, where the rust keeps growing even with a fluid film. Project Farm doesn’t test a lot of good products. I don’t find his testing useful at all. Not very scientific either.
 

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2018 X3 M40i / 2016 X3 xDrive35i
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I understand. My concern is water trapped or rust trapped under any material, where the rust keeps growing even with a fluid film. Project Farm doesn’t test a lot of good products. I don’t find his testing useful at all. Not very scientific either.
I think his testing is quite practical. And I think what we're trying to relay is that in addition to his testing some of us have years of experience using the product and have seen the impact it has on preventing new rust and slowing down old rust.

The oil coatings push out moisture. It doesn't trap moisture under it...it quite literally cannot as the oil migrates to the metal and pushes the moisture out.

A couple of weeks ago I cleaned up uncoated rusty rotors removing the rust. After washing the car the rust immediately started to come back so I decided to go ahead and apply some fluid film to the hub portion of the rotors and then washed it off after it sat for a day. The hub soaked in some of that lanolin oil and after a couple of washes it's no longer rusty around the hub. The stuff works.
 

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If this is what keeps you from sleeping you haven't had any real life issues YET!
SURFACE rust, your in the Midwest, they use salt because of the temps seen in the winter.
Washing all the time, activates the salt on the metal.
Wash when temps will stay above 32 for a while.
I do a heavy spray in the fall of Kroil, undercarriage, springs etc. It helps slow down the "ATTACK"
SILICONE all door seals/rubber to keep from having them freeze to door during freeze/thaw times that ice everything up.
I have been "treating" the underside with Kroil for years, I will get light surface rust, but can work on and remove parts without breaking knuckles and I know this does work.
 

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That’s awesome. I’m glad it works for you. I’m in the South East so rust is not a big issue. Bad drivers with no insurance is the problem.

Luckily, I don’t have to resort to all that oil on my undercarriage. But will keep it in mind if I move back East. Uurrrgg. The snow and salt eats cars to the bone!! They literally disintegrate. Why can’t Subaru do better with rust prevention??
 
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