In the Airstream universe you will find a curious phenomenon: passionate debates about caulk.  That’s because the constant movement of all parts on the road causes all travel trailers to leak eventually, and a good caulk 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-caulked 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-caulking. They’re busy re-financing.

Another reason Airstreamers love to talk caulk is the aluminum construction of an Airstream.  Silicone caulks don’t adhere as well to aluminum, so we tend to go for polyurethane caulks that are more expensive but stick like crazy and stay gooey for a long time. But what caulk is the “best”?

For decades, the gold standard of caulk for silver trailers was some gray stuff called Vulkem.  These days Vulkem is just a brand name, applied to various formulations of polyurethane caulk made by Tremco.  I use Vulkem 116 but you can also get TremPro 635 from the same manufacturer, and a few variants. Either way, it’s great stuff.  It’s sticky like hot salt-water taffy and adheres to aluminum like glue, once cured it flexes a lot without breaking its seal, and it’s designed for exterior use (only) so it’s UV-resistant and completely waterproof.

Finding the stuff is the problem.  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.  Locally it’s sometimes available through Home Depot or Fastenal, but here I couldn’t find it stocked anywhere so I had to order it.

You can always go to the local RV store and find products that claim to be roof caulk.  I’ve tried a few and had uniformly disappointing results, especially the “self-leveling” type.  Those RV-store caulks seem to be made for the disposable RV market, because they all break down in a few years under sunlight and start to crack, then leak.  You can go on the roof of my Airstream and see what things (vents, antennas, etc.) have been caulked with the standard products; they’re drying out and starting to crack around the edges.  I will have to go up on the roof sometime, scrape all that stuff off, and re-caulk.  It’s not a job I’m looking forward to.

You can also see the things that were caulked with Vulkem or TremPro.  They’re still soft and pliable, even sticky, after years of UV exposure.  When I removed the old cellular antenna a couple of weeks ago, the caulk exposed to the UV was hardened but intact, and still waterproof. Around the base of the antenna the caulk was still gooey.  Think about that: the antenna was installed in 2005 and the caulk on it stuck to my fingers like fresh glue after eight years!  I’ve got it sitting here on my desk, just because I like to pick it up and marvel at it once in a while.  (Yes, that’s super-geeky but hey, it inspired this blog you’re reading.)

And eight years is nothing.  I’ve taken apart sections of my 1968 Caravel and found Vulkem contained inside joints and seams that was still good.   That Vulkem was probably decades old.

So that explains why I was willing to wait a week to get a tube of Vulkem 116 from an online seller. I used it to seal up the antenna hole for the cellular Internet booster.  The hole was 3/4″ and the cable was only 1/4″ so I used a set of rubber grommets to fill in the gap, plus a 2″ tall rubber “boot” that I found at Ace Hardware.  This formed a small tower on the roof, from which the new antenna cable emerges. Then I caulked the beejeebers out of the whole thing, sealing it completely.

Two days later, the caulk still feels like it was fresh from the tube, and since it cures very slowly (just 1/16″ of an inch per day at 75 degrees F & 50% humidity) I expect it will take at least a week due to our low humidity.  Fortunately, the Airstream isn’t going anywhere for four weeks.  I’ll get back on the roof in late May to check on the antenna cable again, after having towed the Airstream 2,000 miles to Ohio, just to confirm that the seal is flexing appropriately at highway speed and not developing any gaps that could cause a leak. I’m pretty sure it will be fine.

There are a few challenges with using this stuff.  First off, the fumes are stinky and toxic, which may be why it’s not approved for indoor use.  I would not use this in a workshop without having windows open and a fan.  Second, it’s trickier to shape and smooth than silicone caulk, because it sticks to everything.  The old “wet finger” trick that you use with silicone won’t work—it’ll stick to your wet finger.  I strongly suggest wearing disposable vinyl gloves when you work with it, and bringing along a bunch of paper towels for cleanup.

Third, saving the leftover tube is a bit of a pain.  It’s hard enough to obtain that you don’t want to lose the leftovers between jobs.  Some people put it in the freezer, which I think is a bad idea because the smell of the caulk can ruin your food, even through a plastic bag.  I like my ice cream to taste like Oreo Mint, not vanilla Vulkem.

We’re trying an experiment instead.  We wrapped the nozzle in plastic kitchen wrap, and then vacuum sealed the entire tube with Eleanor’s Food Saver.  (Eleanor wants me to mention that this was her brilliant idea, and I’m glad to do so.  If I open the tube in a year and find it congealed then it will be Eleanor’s terrible idea.  But it should be fine.)