There is a product that comes in a orange can in camping departments that is a waterproofer or at least makes things more water resistant. I buy it fairly often because the jacket I use for backpacking as my rain jacket is a nylon shell that was my sons. Its old and when it needs to be washed, I spray it afterwards with this substance. Can’t think of the name right now. Anyway, it adds the water repellancy back to the nylon jacket and I’ve also used it to spray nylon pants that I use for rainwear, and I’ve sprayed tents with it after I’ve had them for some time, and their orginal finish is worn down.
However, I would not spray this stuff on a sleeping bag. It smells to high heaven!! That’s ok when I’m wearing a jacket outdoors, but would not want to be closed up inside my tent with it that close to my face. When I spray tents, I let them air out well before sleeping in them.
Yes, sleeping bags can get a little damp, but I’ve never had one actually get wet, and I live in the humid South. I’ve used synthetic ones for years, and have had a down one now for about 2 years. Just keep them in something waterproof when you’re hiking in the rain. And throw them out on a warm rock in the sun whenever you can, to let any moisture that’s there escape. You body gives off moisture during the night, so air drying it is important to do as often as possible.
This stuff may be similar to the 3M product, but it doesn’t claim to resist spots and dirt, just water.
Swallowtail
Swallowtail