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Old 07-14-2004, 11:36 PM   #1
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Vulcem vs Silicone

Suspect this is going to be a really dumb question but here it is nevertheless - Helped a friend the other week replace his antenna on his '76 25' Tradewind Landyacht.
We used 100% Silicone to seal the outside roof bracket to the roof. About a dozen screws actually hold it in place.
Should we have used Vulcem? And why not Silicone? What kind of mess have gotten ourselves into? A wet one?
bob r...
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Old 07-15-2004, 06:50 AM   #2
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It should be resealed. Remove it, clean and reseal. Not added to the exterior.

Here is the issue. Silicone will stick to the Aluminum for a while....... Then it lets go in interesting ways. Allowing water to get by. The biggest issue is that it looks like it is sealed when it reality it is not.

Sorry to be the bearer of bad news
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Old 07-15-2004, 07:21 AM   #3
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Quote:
Originally Posted by Brohlof
Suspect this is going to be a really dumb question but here it is nevertheless -
Bob I have never seen a "dumb" question on these forums.
But lord knows there are plenty of "Dumb" answers. Be Careful.

thenewkid64 is correct that silicone is to be avoided. In fact just never use it on any Airstream is a good rule of thumb.
For one thing I don't think it is UV resiliant and of course the roof gets lots of UV exposure. But it will stick to aluminum. Just wait til you have to remove yards of it from every seam, you'll know "stick" then.

But I disagree that there is any rush to re-do your antenna job. Depending on how well you applied the silicone, it should be watertight for quite some time to come. It is a bear to remove silicone. Whethere it is any easier to remove it shorlly after application I do not know. My guess is that after a few days if not hours it is as tough to remove as after a few years. Eventually you can re-do it.

BTW there is one school of thought that silicone contains sodium if I recall corectly. And a thinking that for that reason it may react in some way with the finish and/or metal of an Airstream. It is also not to be used on automotive paint for the same reason, so rumor has it anyway.

Best bet is to just leave it in the hardware store.

Oh and do some thread searching here regarding Vulkem as there are different formula numbers out there. It's such a PITA job that it makes sense to only do it once when possible as newkid64 advised.

http://www.airforums.com/forum...light=caulking
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Old 07-15-2004, 07:25 AM   #4
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"But I disagree that there is any rush to re-do your antenna job. Depending on how well you applied the silicone, it should be watertight for quite some time to come. It is a bear to remove silicone. Whethere it is any easier to remove it shorlly after application I do not know."

I do - it will be easier now. It will not necessarily be watertight now. Silicone has the curious property of adhearing tenatiously to aluminum without necessarily making a watertight seal. In cleaning it off you have to be sure and clean it ALL off. It wants to leave behind a thin film that will prevent a proper sealant like Vulkem or Sikaflex from holding.

Mark
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Old 07-15-2004, 07:48 AM   #5
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Being that I'm from Mizzou...please Show Me

Quote:
Originally Posted by j54marl
Silicone has the curious property of adhearing tenatiously to aluminum without necessarily making a watertight seal.
Mark I will yield to your expertise.
But the above statement just doesn't seem to "hold water" from a logic stand point. If the stuff is tenatiously adheared, just where does the water penetrate? There would need to be microscopic tunnels under the silicone or perforations in it.
I know it is the watertight sealer often used on aquariums but admittedly I have never had an aluminum aquarium.
Could you cite some data?

I will search my warehouse today for a tube of silicone to see if it states not to use it on aluminum. (If it does just goes to show how few Airstreamers read instructions). I'll also search my old Sweets files to see if I can find a customer service number for a silicone supplier. As I am past the aquarium owning stage of life I would love to see the stuff disappear from the universe.
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Old 07-15-2004, 08:42 AM   #6
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Albany Brake clean at Autozone will cause the Silcon to come up. you want to pull up as much as possible before using this. Then dampen a paper towel and get just that area with silicon till it starts wrinkling.

Here is my disclaimer. Our Coach predates any clear coating. I cannot guarantee this will not coause a problem with clear coat. I had a LOT of silicon all over our 59 from the PO and I found this worked well to get rid of it. I then went back with Vulkem.
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Old 07-15-2004, 09:18 AM   #7
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I have heard from a few people in the position to know (such as Inland Andy) that it isn't so much that silicone doesn't stick to aluminum, so much as that it, in short order attracts dirt and dust, which turns it to a nasty brown/gray color, as well as that, when you do remove it, it has a tendency to leave a residue that makes it harder for any new adhesives to bond to the surface. I'm sure if you do a few searches regarding the use of silicone on this forum you'll find the information I'm referring to.

Here's a thread with some explanation.
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Old 07-15-2004, 10:02 AM   #8
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Quote:
Originally Posted by roadeo
Mark I will yield to your expertise.
But the above statement just doesn't seem to "hold water" from a logic stand point. If the stuff is tenatiously adheared, just where does the water penetrate? There would need to be microscopic tunnels under the silicone or perforations in it.
I know it is the watertight sealer often used on aquariums but admittedly I have never had an aluminum aquarium.
Could you cite some data?
First, let me note that my references to "silicone" are all to the common multi-purpose silicone sold at home centers and lumber yards. There apparently ARE specialty silicone products designed to be used with aluminum. I've never had occasion to use any as the polyurethane products (Vulkem, Silaflex) are about the same price and work so well.

My source of data is 25 years of experience as a property manager who has again and again had to clean up the mess left by carpenters (and occasionally roofers) who think contruction silicone will seal leaking aluminum gutters. In more than one case we have had to tear off the old gutter and replace - it was too time intensive to deal with. As for being tenatious, apply some to aluminum, let it set a few years and just see how easy it is to remove.

In re: aquariums. The corallary to your arguement would be that since silicone has such weak adheasion to glass (it just peels right off!) it can't be used to waterproof glass items.


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Old 07-15-2004, 11:53 AM   #9
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My understanding is that silicone kinda picks and chooses where it will stick on aluminum - therefore it may look like its sealing in some places, whereas in other its not - thats the reason it looks stuck but isn't - hope that makes some sense

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Old 07-15-2004, 05:51 PM   #10
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Spcial aluminum silicone?

I was in the paint department at HD the other day looking for some caulking material to use to re-attach my exterior wheel wells to the inside edge of the body. The 1973 service manual spcifically says to use silicon or equivalent caulking for that purpose. I happened to see what appeared to be a new product by GE. The claim was that it was a specially developed silicon for use on aluminum. It supposedly was designed to be used on aluminum used for roofing, gutters and RV's. It also happened to be a silver color. I bought a tube thinking that it met the qualifications stated in the service manual. I have not yet used it on my wheel wells but intend to tomorrow. I understand that this particular application on my AS is different than for use on the exterior of the body but I still wonder if I should give it a try or not. Any thoughts?

Malcolm
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Old 07-15-2004, 09:03 PM   #11
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Malcolm I have used this product on the trim on my unit. I have read all the stuff about not using silicon, but this stuff is made for aluminum looks good and does a great job. it is on the lower trim joint and the body is painted below the trim. I would use it again just because it works and it looks good.
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Old 07-15-2004, 11:41 PM   #12
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Si Vs Vulcem - Thanks for the feedback

Forum folks - Thanks for all your replies. Think will leave the Si for now as the outside antenna mounting plate has a lot of area for the potential leakage path of the water to go thru before it can get into the thru hole to the cabin.
If had to do it over again would definitely go the Vulcen route and will use it in the future for sealing.
FYI - Am a just retired Adhesive Development Chemist (37 yrs / 3M) focusing in the Electronics and Aerospace Industries. There we typically use plasticized epoxies, acrylics and polyurethanes for sealing. The strong suit for the silicones however over the above is the retained flexibility at the extremes of both hot and cold and since the TCE (thermal coeifficient of expansion) of Al is quite high the Silicones being able to move are typically a good fit.
One of the problems with bonding to Al is that it's surface oxidizes very quickly in air causing problems with the good bond you think you have (had).
However with most bonding applications on an Airstream you are actually bonding to an organic protective coating and not the Al so I think their use is ok...
Sorry to get so technical....I'll go back to being retired now!!!
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Old 07-16-2004, 12:41 AM   #13
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As I said above I would go and look on a tube of silicone in the warehouse.
Well I found one:
Dow Corning Brand Multi Purpose Silicone Sealant Trade Mate II
According to the tube it has "Outstanding Adhesion" to;
Glass, Metal, Non Oily Woods, Aluminum, Plastic/Rubber, Fiberglass,Vinyl, Porcelain, Ceramic Tile. 24 hour cure rate.
As I suspected.

However I will still not use it on my Airstream for any purpose that I foresee.


To Brohloff:
Welcome aboard and please don't be a stranger to the sealant discussions.
You may be the only one who can determine the differences in the many varieties of Vulkem, and the mystery products Alcoa Gutter Seal and Parbond. I suspect we all make too much about some products differences, but the manufacturers do a bad job of making it clear. Any chance you are kin to the Lester Rohloff family? He was a famous preacher in Texas I believe, (now deceased). He had childrens homes in Missouri and elsewhere.
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Old 07-16-2004, 04:32 AM   #14
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Quote:
Originally Posted by brohloff
...One of the problems with bonding to Al is that it's surface oxidizes very quickly in air causing problems with the good bond you think you have (had).
However with most bonding applications on an Airstream you are actually bonding to an organic protective coating and not the Al so I think their use is ok...
Sorry to get so technical....I'll go back to being retired now!!!
bob r...
Bob,

Interesting information!
I wonder if one of the issues of the silicone not bonding properly may be because it is not compatible with the clear coat, or causes it to chemicaly break down? I also know that silicone does not want to adhere if there is any surface contamination. FWIW I work in the commercial roofing/sheetmetal industry. NONE of our manufacturers will allow silicone for their products. All of them specify a butyl or urethane caulk, (depending on application) typically Sikaflex, Dymonic, NP-1, and the like. These manufacturers provide 20+ year warranties and the caulk is the weak link in the system.

Aaron
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