Sealants — A Sticky Subject

In the Airstream universe you will find a curious phenomenon: passionate debates about sealants. That’s because the constant movement of all parts on the road causes all travel trailers to leak eventually, and a good sealant is our first line of defense against rain penetration. Rather unfairly, Airstreamers get more of this debate than owners of other brands, mostly because Airstreams have been around for so long, and so many of them remain on the roads after decades of use.

An Airstream might easily be re-sealed a dozen times over its lifetime. In contrast, “disposable” cheap travel trailers tend to get chucked into a landfill when they start to leak because they’re starting to fall apart too. You don’t hear the owners of those talking quite so much about re-sealing. They’re busy re-financing.

The aluminum construction of an Airstream is another reason Airstreamers love to debate sealant. Common silicone sealants don’t adhere as well to aluminum, so we have to use polyurethane sealants that are more expensive but stick like crazy and stay gooey for a long time.

By the way, we use the term “sealant” because “caulk” isn’t flexible, whereas the sealants used on an Airstream are elastomeric, meaning they can stretch without breaking their seal as the travel trailer moves down the road.

Sealants, then and now

In the old days the general-purpose sealant of choice was called Vulkem, and you’ll still hear vintage Airstream owners talk about it. Like the modern sealants, it had a marvelous ability to seal gaps tightly, stick to aluminum, and remain slightly tacky beneath the surface for many years.

These days it has been supplanted by a more modern formulation that carries the same name, as well as a few other new products. Most of the recommended modern sealants have certain characteristics in common: they are sticky like hot salt-water taffy and they adhere to aluminum and plastic like glue. Once cured, they flex a lot without breaking their seal, and they are designed for exterior use (only) so they are UV-resistant and completely waterproof.

SealantAs long as you choose a sealant that meets those criteria, you can use any particular one that fits your needs. Popular choices include TremPro 635, Vulkem 116, Sikaflex 221, ParBond, AdSeal, and others. ParBond is thinner than the rest, comes in a silver color, and creates a “rubberized” seal that’s good for tight spots, so it’s typically used on small jobs. The others are great for roof work (they come in colors, so get white), and larger areas of coverage.

Where to shop

Finding appropriate sealant is sometimes difficult. Lots of online sellers offer it but since a tube is anywhere from $6-14 and shipping tends to add $7-9, you want to get it locally or combine it with another order to duck the shipping charge. If you are in a rush to fix a leak, check hardware stores for a construction sealant that is guaranteed waterproof, UV stable, approved for outdoor use and which adheres to aluminum, painted surfaces, and plastic.

You can always go to the local RV store and find acceptable alternative products, but in my experience white-box RV store products seem to be made for the disposable RV market because they often break down too quickly and start to crack, then leak. By comparison, a really good polyurethane sealant can stay sticky, waterproof and pliable for decades, in places that aren’t exposed to the sun.

cracked sealant
Cracked sealant

Sticky situations

There are a few challenges with using this stuff. First off, the fumes are stinky and toxic, which means you have to beware of the fumes if you are lucky enough to have a garage to work on your Airstream.

Second, it’s trickier to shape and smooth than silicone, because it sticks to everything. The old “wet finger” trick that you use with silicone won’t work—these sealants will stick to your wet finger. Wear disposable vinyl gloves and bring along a bunch of paper towels for cleanup.

Third, keep in mind that the good sealants tend to cure very slowly. For example, Vulkem cures at the rate of just 1/16″ of an inch per day at 75 degrees F & 50% humidity. You’ll want to allow some time for a good cure before exposing the new sealant to weather.

Saving sealant

It’s hard to save the leftovers. Once you’ve opened a tube of sealant, take some care to re-seal it well for storage. There are many techniques to try to seal the tube, such as putting a golf tee in the opening, covering with the tip of a rubber glove, using food saver vacuum bags, and commercially-available storage caps of various designs. Hot glue on the tip works well.

Don’t freeze it—that method can cross-contaminate other items in the freezer (your ice cream might taste funny after a while) and the manufacturers of the sealants generally don’t recommend storing at that temperature.

Whatever you do, don’t expect opened sealant to still be usable in a year. Even if you take tremendous effort to close up the tube against outside air and moisture, it will probably cure to a solid lump in a few months. Parbond seems to be the exception. It lasts for years just by replacing the cap.

Safety first

The sealants discussed here are fairly safe but you should take a moment to read the fine print on the tubes before using them. Avoid skin contact or breathing of the vapors in a confined space. (That also goes for chemicals used to clean up afterward.) There are recommended temperatures at which you should apply sealants, and recommended techniques for application, cleanup, and storage.

Sealing the Airstream against leaks really isn’t rocket science. Generally the sealants last a long time, so the old days of climbing on the roof to “re-caulk the seams” every year are over. When replacement is eventually needed, making a good choice of product will be a key part of keeping your Airstream dry and happy.