When using Olympic rivets on the exterior of an Airstream, Spartan or any other travel trailer, we recommend using the type WITHOUT washers in almost all applications.
We believe it provides a better seal over time, and the rivet sits tightly against the skin for a more original look. In addition, there is some risk with the washer that the sharp edge of the hole in the skin may cut the rubber washer during road vibration if any part of the washer slips into the hole. If that happens, your rivet will be loose and no longer be water tight.
No matter which version you use, always squirt a little dab of Trempro 635 or other quality urethane sealant in the hole and on the shaft of the rivet before installing it. This will provide the best watertight seal. If done correctly, a little extra Trempro 635 will squeeze back out when you're installing the rivet. Just wipe it up immediately with a clean rag and a little mineral spirits. Using this method WITHOUT the washers will provide an excellent seal.
It is handy to keep a small number of rivets WITH washers in your toolkit when on the road in case you need to make a repair and don't have Vulkem on hand. Washers can always be removed before installation.