Great question — answering as the UphillTech team:
There are many commercial products to "waterproof" cotton, such as Nikwax Cotton Proof , Scotchgard and Kiwi Camp Dry , my personal experience has been that these treatments are temporary (often removed by the first wash) and not too effective (direct contact with a wet object or impact of raindrops penetrates the garment). That said, one of these products might be good enough for casual wear of the shirt, being reapplied as needed. Some fabric softeners also afford a bit of water repellency; you'll need to test one. Waxing cotton does make it waterproof, but it also makes it a fire hazard, as well. An ember from a campfire can turn a shirt or tent into a torch! (BTW, I've yet to find a...
Full answer by DrMoishe Pippik: https://lifehacks.stackexchange.com/a/15922 (CC BY-SA 3.0).
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