Does Silicone Sealant Cause Rust?

I am just now looking into that. This is a comment from another forum. And it happens to be another Mercedes forum.

"It has been mentioned previously that the use of silicone sealants on painted metal can cause rust and the recent discovery of a silicone like sealant on my car has concerned me. After some reading I have found that there are two types of silicone sealants; neutral (non-corrosive) cure silicone and acid curing silicone.

The acid curing silicone is corrosive and would etch into the paint and ultimately cause rust. This is usually designed for use on glass.

The neutral cure silicone is non-corrosive and I assume will not etch into the paint surface and should not cause rust. This is the type of silicone used for metal roofing etc. and one manufacturers website lists the uses including "Weatherproofing caravans and trucks".

So, is the use of a neutral (non-corrosive) cure silicone sealant bad for cars?"

Read some of the other comments.

https://forum.w116.org/auto-torque/silicone-sealants-and-rust/

"When I contacted a Permatex person some years ago for advice on a RTV in a non-automotive application (galvanized vent), he recommended their Ultra Black as one of their better/best RTV's for non-corrosion, adhesion, oil resistance, etc.. A peek at their product data sheet will support this."

You will have to read post number to see that the data sheet on the Permatex Ultra black has that it is non-corrosive.

From post 3: "Yes most RTV sold most places is an acetic acid cure. You can find alcohol cure RTV however, those are sometimes labeled as potting compounds. I bought Dow Corning 838(?) once from McMaster-Carr to seal up a trailer wiring connection."

From post 11: "Any automotive RTV spec'd as "Sensor Safe" is not acid cure so will be fine, including Permatex ultra blue."

https://bobistheoilguy.com/forums/threads/is-rtv-silicone-corrosive-to-painted-metal.347520/

So, I am looking at my dollar store tube of silicone sealant made by Pro Seal.

I don't see anything written that has non-corrosive or corrosive on it.

I have used 'ultra' type silicone sealant from Permatex and it smells different from the Permatex regular silicon sealant and their regular silicone sealant smells like the Pro Seal silicone sealant.

If we take Permatex's word for it the 'ultra' type Permatex is non-corrosive.

As the one post mentioned ask the maker if the product is non-corrosive or not.

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84 300D, 82 Volvo 244Gl Diesel

But if you use a sealant or anything where water gets trapped in it, that causes rust.

Did you read all of the responses in what I posted?

It was said that the acidic cure silicon can I guess in some cases eat off the paint. If that is the case it can do that with or without water present. That I what I got from what I read.

In any event persons that read the whole of it will make their own decisions. And that is the reason I posted it to give info so persons could decide on their own.

I neither promote nor detract from the use of silicon sealant. I just make my own personal choice with it.

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84 300D, 82 Volvo 244Gl Diesel

Interesting and eye opening post!

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Acetic acid is released during the cure of many silicone sealants. You can detect the vinegar smell when you use it.

There are other types of silicone with a different cure chemistry that does not produce acetic acid. Sometimes called neutral silicone.

The first post covers all of this. I can't think of any place I would need or want to use normal silicone sealant on my cars?

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

On my 84 300D (I did not know which one I was using till I looked it up) when I change a valve cover gasket, I degrease the channel in the valve cover with brake cleaner. I put a bead of the sealant into the channel all the way around and I apply the rubber valve cover gasket. After the silicon cures it will be glued to the valve cover.

I just wipe the oil off of the cylinder head where the valve cover seals leaving an oily film. I install they valve cover with the gasket and just snug the nuts slightly and let is cure for many hours before I tighten it down.

Doing that I have never had that leak using victor reins gaskets as some have had with the same make of gasket.

I have also used it on eroded aluminum coolant hose nipples where there is pitting. Get all of the white corrosion off of the part down to the bare metal and put a good coating of the RTV sealant over the sealing area and slide the hose over it and tighten it down. Let is cure before filling the system with coolant.

When I replaced only the oil cooler hose, I put RTV on the degreased nipples, slid the hose over and applied the worm gear claps. That glues the hose to the nipples.

When I installed the ne turbo oil drain grommet I also used RTV silicon sealant on it.

I only use it for particular things.

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84 300D, 82 Volvo 244Gl Diesel

I have in past used a few small dabs of a contact type adhesive in the groove just to hold the Victor Reinz gasket in place while installing the cover. However, OE gaskets were a tight fit and did not need an adhesive to keep them in place during installation.

I avoid using general purpose acetoxy cure silicone sealants anywhere on my cars. Same for marine applications because silicones shrink while curing and then leak. They are more a caulk than a sealant.

On my W210, a sealant is specified on the breather plates that are part of the valve covers. I used Reinzosil on those which is a similar product to the MB & BMW OE sealants, but less expensive. I use it on other critical jobs

It has no outgassing and is quite amazing in that it can be applied on the outside of a leaking joint and it somehow stays in place.

https://www.reinz-industrial.com/EN/PRODUCTS/Sealing-Compounds/REINZOSIL.aspx

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I use what works for me. I have had the 84 300D since 2007 and I still do the valve cover gasket in the manner described. It just works for me.

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84 300D, 82 Volvo 244Gl Diesel

I've seen guidance not to use silicone sealant on sheet metal for the reasons cited. This and paint adhesion is why body seam sealer exists. The gases that stuff gives off are bad if not worse, but it won't rust paint, nor will it reject paint.

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1983 240D 4-speed manual

No argument about doing things your own way if you see fit.

As an amateur DIYer, I try to learn better ways of doing things as I go along. These forums can help.

By the way, I have had my 85 300D since 1990. 34 years! - still looks and runs well! But it has needed regular maintenance and repairs. Still use it as daily driver, despite other options.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Seam sealer is a great product for smoothing bodywork joints and repairs.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

When I work at home, I use what I have at home first and what works for me. And what is cheapest. Before the 99 cent stores closed that is what it used to me for a tube.

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84 300D, 82 Volvo 244Gl Diesel

I'm trying not to be confrontational here, but this is yet another example of Diesel trying to do the wrong thing because he thinks he knows better.

Most owners shouldn't try to reinvent the wheel as there's already a tested way of doing things and a wide variety of acceptable materials to be used. I can't prevent anyone from "repairing" their oil cooler hoses with screw clamps, but I can sure advise against such lunacy.

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Onus probandi incumbit ei qui dicit, non ei qui negat

I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

You can give your personal opinions. Giving an opinion does not make you correct.

All I did was post information on the use of silicon sealant and said what I have used the stuff for. As I said people can make their own choices.

If I do something and it works for me how is that the wrong thing?

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84 300D, 82 Volvo 244Gl Diesel

There are people who use drywall screws to build wood structures. "It works for me" is just another way of admitting that you're doing it wrong and you don't care.

I speak up on here because you aren't simply doing it wrong, you're also sharing it with other in an attempt to advise them of the legitimacy of your option.

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Onus probandi incumbit ei qui dicit, non ei qui negat

I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]