Silicone caulking and aluminum

Tom's RV Discus Message Board: The Airstream List Discus Message Board: Topics N - S: Silicone caulking and aluminum
By Bassman on Thursday, January 20, 2000 - 08:15 am:

RICK & CAROL BELL wrote:

I have read, many times, on the list that siliscone caulking should not be use on aluminum. I am looking at a tube of "GE silicone II" that states "Superior adhesion to : woods, metals (will not corrode metals), concrete & masonary, ceramics, glass & most plastics. I would think that the skin of an A/S would be covered by this list of surfaces. Am I missing something?

No argument that it will adhere to your Aluminum. But the word of metals versus metal might allow for interpretation that it will not corrode "certain" metals.

Here's what another Airstream owner who also is in the business of polishing and repairing Aluminum had to say about Silicon Caulking on Aluminum.

Al, you wrote:

There are various commercial preparations that soften the silicone caulk after 8-12 hours. They work very well. You can buy them at Marine stores or at Home Depot. About $2.50 for 12 ounces. It goes a long way. I use them to replace the silicone caulking in our bathrooms after the caulking is begins to look ratty. Look it in the paint department. I do not know if they work on caulks other than silicone.

Al

They DON'T make boats out of Aluminum! CAUTION: Test EVERYTHING you purchase. ALWAYS look at labels & stay AWAY from Silicones - PERIOD!! Just some of other things that DO NOT mix - or are No-No's with Aluminum. - Teflon, Ammonia, Salt, Salt Water, Salt Water Spray (if you live anyway near an Ocean, Sea, etc., and have Fog - you have Salt Water exposure! The further South you go, the worse the damage to Aluminum is, with the salt spray and the added heat of the UV Rays, smoke a clearcoat in record time!)

And just HOW do you, "test"? Get yourself some Aluminum. Wash it with, HOT water & Dawn degreaser dish soap and dry thoroughly. Put caulking on Aluminum, put in oven at approx. 90 degrees (or lowest temp., you can set on your oven) for one hour. Put outside, overnight. NOW you can apply chemical to remove it. Once removed, bring aluminum BACK inside and let it come to room temp., then back outside overnight. OK - now put the Aluminum in the DIRECT sun rays for about 30 days. Then look at it with a magnifying glass. Look for pits, line, stars, stain marks. If ANY of these are present - DO NOT use the product on your Airstream!

Also, when using a "scraper" to remove ANY type of caulking from an Airstream - ONLY use a PLASTIC scrapper! And then use it VERY, VERY GENTLY! For EVEN a Plastic scrapper can damage the skin if not used CAREFULLY!

Later,
Jim Weston


There are more archives on this and many other topics at /Streamline/Onlinelist/Online.html

BASSMAN

By Charlie/Betty Burke on Thursday, January 20, 2000 - 08:19 am:

I am looking at a tube of "GE silicone II" that states "Superior adhesion to : woods, metals (will not corrode metals), concrete & masonary, ceramics, glass & most plastics. I would think that the skin of an A/S would be covered by this list of surfaces. Am I missing something?

If everything sold did everything the package says it is supposed to we wouldn't need half of what's out there. The experience over the years has been that silicone based sealers do not have the longevity to adhere to the outer aluminum shell compared to other products. Whether it is heat or expansion or clearcote is not clearly know. Experience is that silicone will rapidly separate from the surface and while appearing to adhere it is in fact lifted off the surface allowing dirt built up and moisture to penetrate the joint.

Charlie

By RICK & CAROL BELL on Thursday, January 20, 2000 - 08:20 am:

I really do appreciate your reply, but I still am not sure what material I should use for caulking. I have been in the ship repair business for 34 plus years and the US Navy does have aluminun ships and boats. We used quite a few in South East Asia and they had silicone in several areas. These vessels, however were fabricated from a marine grade alloy, and airstreams probably are not. Is there an acceptable caulking material for airstreams?

By Bassman on Thursday, January 20, 2000 - 08:21 am:

I use a polysulfide caulk made by Boatlife. Sold at West Marine Stores for one. It can even be applied underwater. In fact water is what cures it. Never gets brittle and can be cleaned up with kerosene.
http://www.lifeindustries.com/0001.htm

make sure it's not the Silicone They also sell.

BASSMAN

By Susi and John Burchard on Thursday, January 20, 2000 - 08:24 am:

Just a couple of comments on this, I may have made them when Jim's original post appeared long ago, can't remember now ...

1) silicone caulk generates acetic acid while curing, but not afterwards. I've not myself used it on metal, but when used on clean inert surfaces (glass) it is very strong and very durable. We used it to make LARGE aquaria for tropical fishes (my research objects for many years) ... I mean, pieces of plate glass six or eight feet square and thick enough to withstand the water pressure of a tank that size ... and they lasted forever, in spite of sea water and warm temperatures etc.

2) they do build boats out of aluminum, a great many of the fast crew boats used in the oil industry are aluminum hulled. Not rowboats, but 80 or 100 feet long and designed for hard fast usage under tough conditions (we had a lot of them in the Arabian Gulf when I worked in Saudi Arabia). Also, of course, in a more familiar context, practically all the countless outdrives and outboard engines have their lower units at least (the part in contact with sea water) made out of aluminum. The key to that is the use of sacrificial anodes (pieces of a metal more electropositive than aluminum, so it corrodes instead of the aluminum under galvanic action, bolted on in strategic places and replaced regularly before they are completely eaten away) or in a more sophisticated version, something called impressed-current cathodic protection ... you have a sensor, a platinum electrode, in the water, sensing the voltage generated by the aluminum hull, and a solid state regulator which applies exactly the opposite voltage ... Mercruiser (and I suppose the other manufacturers) even makes such a kit for ordinary pleasure craft, I installed one on my own twin-Mercruiser Bertram work/fishing boat. It spent many years in one of the world's most corrosive marine environments (the high salinity, high temperature waters of the Arabian Gulf) and suffered no corrosion to speak of.

Hard to do that with a travel trailer, though, and I'm sure Jim is speaking with the voice of experience. Aluminum is a soft metal and easily damaged by any kind of harder or sharp material. It is also one of the most electrically active metals, and therefore very subject to galvanic corrosion.

John

By Susi and John Burchard on Thursday, January 20, 2000 - 08:25 am:

Experience is that silicone will rapidly separate from the surface and while appearing to adhere it is in fact lifted off the surface allowing dirt built up and moisture to penetrate the joint.

I have an idea about that (shoot it down if I'm wrong!). Bare aluminum when exposed to air quickly forms a "skin" of hard aluminum oxide which then inhibits further corrosion unless there is something there that combines with the oxide (acid rain for example). Silicone polymer is probably at least somewhat permeable to oxygen. So even when you apply it to a clean freshly polished aluminum surface, the oxide layer will in time form underneath the silicone. The oxide is hard (corundum is one of the aluminum oxides, it is a well known industrial abrasive and the next hardest naturally occurring material after diamond) but it is also finely granular, almost powdery in consistency and probably does not give the silicone a good "grip" on the surface. Then moisture gets in underneath, and ...

Am I completely off track here?

John

By Richard P. Kenan on Thursday, January 20, 2000 - 08:26 am:

Not off track so far as I know - that's the story I heard years ago about aluminum - which made me wonder what the stuff everyone calls "oxide" really is. Is it some kind of corrosion that manages to get past the aluminum-oxide layer? I think alumin-pxide is transparent (unless dyed), at least in such thin layers.

- Dick
(5368)

By Susi and John Burchard on Thursday, January 20, 2000 - 08:27 am:

Maybe that visible "oxide" is aluminum hydroxide, formed by interaction of the oxide with water? Geochemists ought to know this stuff, a large part of the earth's crust consists of mixtures of aluminum oxides and silicates.

John