Beeswax.
This rants a bit but has some photos unlike most discussions, and has some other useful hints:
https://blackpowderguide.com/how-to-waterproof-a-cap-and-ball-revolver/
I had never heard of better nipples providing better waterproofing, but it's good info.
Note that many candles and other formerly-wax things are not great waxes so old school methods don not always work with modern materials. I also think dripping from a lit candle is sorta asking for it with BP but maybe I am paranoid. Anyway, go out of your way to buy proper beeswax for this purpose alone. I also have some on hand for woodworking etc. I like it.
If you have beeswax lube (tallow and wax) that can work also, but make sure it's stiff enough to stay in place.
Also: this will be more water resistant, and is likely a good idea even in super humid areas. If actually pouring rain, you also want to keep the gun from direct exposure. Keep under a coat, in a flapped holster, etc.
For long arms you can get lock covers (or action covers, etc). These are authentic, period correct things, okay to use even if trying to stay that way. Most are leather, some were (not sure if anyone makes such today) waxed fabric. Make sure it is treated to be weatherproof, dry it properly between uses etc. Also look at how it fits, and make sure you don't carry so that water runs into it. If not trying to be period correct, I bet you can easily sew (or have sewn) the same out of modern fabrics, with modern fasteners to be quicker releasing as well.
I have also seen covers themselves once or twice, waxed onto the action. But that may be a bit much, will leave a rim to be removed later on if you want to keep the gun pretty