Sealing an Airstream
They all leak. If it doesn't leak now, it will leak in the future. Airstream restoration and ownership involves lots of sealing and re-sealing to prevent leaks.
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I highly recommend checking every single rivet on your Airstream, one by one, and sealing or replacing the bad ones. This is a job you can finish in a day, and you will almost certainly find an alarming number of leaks you didn't know about.
To check a rivet, stick a suction cup over the rivet. If the suction cup sticks, the rivet is good. If it doesn't, the rivet needs to be resealed. When you find a bad rivet, the suction cup will often pull a bunch of water out of the hole. Fun! Disconcerting!
If you have the skins off, you can often rebuck bad rivets to seal them back up, but the approach I prefer is to remove the rivet and replace it with a wet bucked rivet. It's a good idea to remove every Olympic rivet you find and replace it with a wet bucked rivet, since Olympics tend to leak over time. The buck rivet kit from Vintage Trailer Supply is a good investment if you're doing any major work on your Airstream. To wet buck a rivet, put a dab of Sikaflex 221 on the hole before you insert the rivet, then buck as normal. Once it's bucked, use a clean rag with a bit of carburetor cleaner to remove the excess sealant.
If you don't have access to the back side of the shell, you can use Captain Tolley's Creeping Crack Cure to seal up bad bucked rivets. Add the sealant and let it dry, then test it with the suction cup again. Repeat until the suction cup sticks. For bad Olympics, it's better to drill out and reinstall with a new Olympic than try to reseal. Use the Olympic Rivet without the neoprene washer, and install it wet.
Window and Door Gaskets
After decades of use, the window and door gaskets on vintage Airstreams need to be replaced. Check Vintage Trailer Supply for correct window and door gaskets for your model. To remove the old gasket from aluminum door and window frames, I suggest using a sharp razor blade to remove the bulk of the gasket material. Then soak what's left in goo-gone, scrub with a soft-bristled stainless brush, and repeat until clean. The brush will scratch the aluminum to a "brushed" look, but since the area is behind a gasket anyway, you'll never see it. Be sure to clean the goo-gone off with carburetor cleaner before trying to apply the new gasket material. Don't use this method anywhere other than behind gaskets, since it will scratch the aluminum.
If your Airstream has cast aluminum plumbing vents, the vent pipe gaskets need to be replaced as well, and I'd suggest adding a little sikaflex under the gasket as added insurance.
Window, Door, and Vent Frames
The sealant around the window and door frames also to be removed and replaced periodically. Old Vulkem sealant is easiest to remove without damaging the aluminum using carb cleaner, a bunch of rags, and this 3m radial bristle disc. Run the disc over the sealant to break the surface, soak it with carburetor cleaner, then run the disc over the sealant again. Repeat until you've removed most of the old sealant, wiping up with your rag as needed.
Use a caulking syringe to inject a nice clean bead of Sikaflex 221 into the channel you just created. I prefer Sikaflex over Vulkem 116 or its successor, Trempro635, because Sikaflex isn't sticky when it's cured. I am not a fan of the aluminum tinted gutterseal, since I didn't find that it "wicked" into seams like it's supposed to, and it's impossible to tool once applied.
Never, ever use silicone caulk on an RV. It will fail quickly, and is an enormous pain to remove. If the previous owner used silicone on the shell, you'll want to remove it with several applications of silicone caulk remover.
Lots of folks recommend using painter's tape to mask off clean lines around your caulk lines. I've never had much success with this method. Instead, I wipe any excess off with a finger, then repeatedly wipe with a carb cleaner soaked rag until all that's left is a perfect line. The caulking syringe helps a lot with ensuring you don't add way too much sealant.
If you're adding a new vent or other item to the Airstream shell, it's a good idea to use butyl tape under the lip against the shell, then seal with sikaflex around the perimeter.
Lots of folks like to break out the inner pane of their double-pane windows to "fix" them. I find that single-pane windows in an Airstream tend to accumulate a lot of condensation when it's cold outside, which is not good for cold weather camping. The double-pane is worth preserving if you're able to.
If you can get the panes apart without breaking them, clean them up and reseal them with polyshim butyl tape. VTS recommends this product for Vista View windows, but it also works for wing windows and some double-pane side windows. Depending on the window, it might be a good idea to add a bead of sealant around the perimeter, though beware, this will make it much more difficult to separate the window in the future.
Despite all of your hard work, you will have leaks. Unless you spring for an impervious subfloor material like Coosa, you're going to want to seal your subfloor.
I encased each sheet of our plywood subfloor (exterior grade BC plywood) with marine grade West System Epoxy. It took two and a half of the gallon kits with the 205 hardener to do both sides and the edges of the subfloor with two good coats.