You should check out Justin's video about water entry. Basically if you ride in the rain a lot you won't be able to prevent water from getting sucked into the motor, since even if well sealed the negative pressure resulting from cold rain cooling a warm motor will suck water in through the wiring acting like a straw. You've got 2 options:
1. If you can tolerate the occasional drip of oil, put some ATF in the motor and pick up the added benefit of better cooling.
2. Put a drain hole at the perimeter of one of the side covers to easily drain any water as well as allow water vapor to escape.
I totally disagree with completely coating the stator with one of those electric sealants. The most I coat is the end lams (the part facing the magnets) and the circuit board for the halls, which I do only if taking the motor apart for some other reason. I definitely would avoid coating the copper, because it already has a high temp coating, and any more will reduce the rate at which a hubmotor sheds heat. I've had ventilated motors since 2011 and have never had any water or debris related related issues with any of them, and that includes several months some years ago where one of the volcanoes was more active cause magnetic dust to settle everywhere.