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You could mix 100% silicone caulk with mineral spirits until it's thin enough to soak into the fabric. Paint it on with a foam brush. I would try this on a couple of scrap pieces first or a small test patch. should make it 100% waterproof. But make sure it can handle the cold temps first as I've never tried it for this. Just an idea.
you can also buy rain gear to fit over your carharts