I’ve been experimenting with finishes and treatments for vegetable-tanned leather in particular. You can’t really weather-proof leather, but you can make it pretty darned resistant, and you should be able to carry one of my notebook covers, have it get rained on (this is Washington, after all), and not have the colors run so long as you wipe it off within a minute or two.
To this end, Ian Atkinson’s video on the topic of finishes was incredibly helpful. (All of his videos have been helpful, actually; I’m still trying to figure out how us ‘Murricans can donate via PayPal.) I bought a large bottle of the acrylic resolene based largely on his results in that video, and after a few fits and starts, I have applied it to a few things (such as the 3rd AD patches) and have been favorably impressed. It’s a pretty water-resistant finish that doesn’t smell horrid (after it dries, anyway).
In addition to some of the other finishes he went over in that video, I also have a few other things of interest floating around. I have a mink oil compound for my old combat boots (from before we moved from black leather to ugly suede) that is supposed to help condition and weather-proof the leather; as a bonus, it smells nice. I suspect it’s roughly equivalent to the Fiebing Snow Proof Paste I picked up recently at Tandy. I also have coconut oil, which I’ve used to moisturize dry leather before; I’m not sure whether it will resist water or not, but it could be worth testing.
Of course, there are some neat things you can do with color and treatments before you weather proof things, as I inadvertently discovered with the linseed oil, and I plan to experiment a little with “running” the colors , just to see if I can come up with something particularly entertaining. Some of the accidental water spots that my SSG notebook cover has acquired (since I took those photos) actually give it a bit more character; I want to see if I can do something eye-catching in a controlled manner. I don’t have the patience, fine motor control, or interest in the more detailed dye work that goes with traditional leather work, but I’m interested to see how else I might distinguish my work from everything else out there.