Good morning Baker, good question! I've been using the same pair of gloves for years now. I cherish them because they are a womens size small. Mann's small gloves are still too long in the fingers and it's so easy to kill bees while moving around in the frames.
So like you, my gloves are worn, very dark in color, and getting a bit stiff in the fingers.
The only thing that I know of that removes propolis is honey. So maybe we could paint the leather part of the gloves with honey, let it soak in rinse, paint, rinse, paint, rinse, and so on?