We use abrasion-resistant mil-spec (military specification) hydrophobic thread. This thread is not widely or readily available and not as easy to work with as other threads. We've been told to avoid the additional expense, but we don't want any weak links. And we routinely do what we're told not to do.
There is never any cotton in WeatherWool.
Our structural thread, Anefil Nylon Dry (Tec 45 and Tec 70) from American & Efird, is non-wicking bonded nylon designed for exceptional performance and seam strength and made in the USA for the US Army. If it wasn't for the Army, American & Efird would not make this thread in America, and we'd have a real problem. Until 2020 or so, it was a little tricky for us to get this thread because A&E didn't stock it, and the minimum production run is 3000 pounds ... so when the Army ordered, A&E made another 30 pounds or so for us. But recently they have been making more and it's no longer a special order, but we still need to order each of our three colors in quantity.
As of April, 2022, we bought quite a bit of the Anefil, just to be sure. I would go somewhat crazy if thread held us back from making garments. And it turns out to have been a very good thing we did buy extra, because lead times (and prices) have taken turn for the worse.
For our non-structural thread, such as used for merrowing, we employ a polyester, American-made thread, also from A&E, that does not absorb water.
As of late 2023, the price we need to pay for this thread jumped about 100%
1 December 2023 --- Ralph