Most of us have heard the phrase "cotton kills," when referring to clothing to be used in cold weather. That's because cotton loses all of its insulation properties when it gets wet.
However, cotton does have some qualities which make it desireable for use in clothing. It's common, mostly inexpensive, wears well, and the right weaves can be very windproof. So, if there is a way to make cotton material less vulnerable to getting wet it's worth giving a try.
Earlier this month I bought a replica second pattern Denison Smock from What Price Glory. The Denison Smock was issued to British paratroopers, commandos, and snipers during and after World War 2. I'd wanted one since first learning of them in the book With British Snipers to the Reich.
The Denison Smock is made of tightly woven cotton twill fabric. It should be pretty wind resistant (I haven't been able to test that yet since it's only September) but as it comes from WPG, isn't water resistant. Wanting to use the smock while out in Fall and Winter weather, I looked around for a water repellency treatment. After a bit of searching on the Internet I ordered a bottle of Nikwax Cotton Proof from Campmor.
Nikwax's directions instruct you to start with a clean article and then apply it with a brush or through washing in hot water. You can do this in a clothes washer but I didn't want to have to get any residue out of my washer, so instead, I filled a 5 gallon bucket with hot water, and added the correct amount of Cotton Proof. I then imersed the smock in the bucket, swishing it around to make sure all the fabric was wet. I then let it sit for about 15 or 20 minutes, swishing the smock around a few times during the soak. Afterwards I rinsed the smock until the water ran clear, and let it air dry.
Today was the first time I was able to test the water repellency. It's misting and 64 degrees F. I put on the smock and took a one mile walk around my neighborhood. I was out in the mist for about 25 minutes. It's a short test, but during my walk the mist just beaded on the outside of the smock. Underneath I stayed dry except for my sweat (it's a bit warm to be walking around in one of these things). After I got home I stuck my arm under the water dripping from my roof to simulate raindrops falling on the smock. While the Cotton Proof kept the mist out, the larger drops went right through, wetting my arm.
Based on this short test, the Nikwax Cotton Proof treatment adds a limited amount of water repellency. It will keep mist out for awhile but rain is going to get through treated material easily. For rain, you're going to want actual rain gear made from modern materials.