I have never done this myself, but you could try thinning some transparent silicone with a synthetic solvent and brush that onto the fabric. The silicone is highly hydrophobic so fabric soaked in it becomes somewhat water proof. This technique is sometimes used for DIY tarps and tents.
zerowaste
Discussing ways to reduce waste and build community!
Celebrate thrift as a virtue, talk about creative ways to make do, or show off how you reused something!
That's a really cool idea! Tbh when I asked the question I'd been thinking of the old 'oilskins' used by sailors in fantasy books (and history) which I think were sailcloth or canvas coated in tar, so this would actually be pretty close to what I had in mind.
One of my favourite YouTube DIYer NightHawkInLight has a detailed video on it. I'm not sure if it will work for your exact case, but I think I would have gone the same way if I was in your shoes. https://www.youtube.com/watch?v=HedRbIsM75M
I think it was more of a wax type of thing, but those become quite inflexible and might not be nice to wear as a jacket.
Oilskin jackets used beeswax cut with a drying oil like boiled linseed to keep it thin and flexible. The same drying oils get used in oil paint because they polymerize into a nice paint-like finish.
You could get flex seal and try painting it on. I just did that on pleather boots. It's not pretty, but it's cheap.
There is the stuff they use to repair Leather/PU car seats, it’s basically stretchy rubber paint that you paint on with a sponge brush. Last time I looked it was more expensive than a pleather jacket. Maybe time this was turned into sofa stuffing, and you replaced it with a genuine leather jacket, that with care can outlive you and your kids.
Probably though I hate to throw something away just because the outer layer is failing. I guess that's why I wore this one till it got like this. I'd like a leather jacket if it fit like this one (and if I got it second hand, I wouldn't be good with buying one new). Someone gave me one once but it fit me like a tent so I had to pass it along.