Brushless motors are naturally waterproof, but like anything made of metal, salt water will play havoc with it over time. I shudder every time I see someone launching or landing from concrete, grass, bitumen or any surface which harbors sand or grit. Always use a landing mat, because although the air from props is flowing downward you'd be surprised at how much small particles are thrown upwards and sucked into your motors and inside the Quad itself. Over the years I've had possibly 15 or 20 Brushless motors brought to me to be repaired. You'll know when a stray piece of gravel or grit has found it's way into the motor. You'll notice a change in the sound the motor makes, That's a minor case. A more serious problem is a seized motor where the piece is wedged between the motor's outer case and the Stator (the piece which spins). That requires a complete dismantling to remove the offending Grit.
Avoid flying over salt water, especially on windy days or very hot days. Moisture evaporates from any body of water and again it gets into the quad itself and the motors. The same with flying in clouds, mist or fog.
I recommend coating your electronics in Conformal spray. I used to coat ESC's and anything else with electronic components attached. With this spray, as a test, I've run ESC's, flight controllers video transmitters underwater after coating them. It puts a waterproof coating over motherboards and components which keeps moisture, dust etc. from interfering with whatever the boards are designed for. Basically it's a silicone based protector and it's simple to apply, just spray the entire board, both sides. I spray it once, leave it for a few hours and spray again changing the angle of spray to ensure every component gets full coverage.
A word of warning. It's best done outside or in a room with doors and windows wide open. One whiff of it and you're in LaLa land.. It knocked me out the first time I used it, it'll take your breath away very quickly, also keep it well away from naked flames. I buy mine from a local communications shop, and it's not that expensive. What surprises me is it's never mentioned anywhere. Something which can save you quad from a miserable death should be used by anyone who flies near water, especially salt water. It may not kill your quad immediately, but salt water will dry out and leave a coating of salt on your electronics. This will eventually eat away at the copper tracks and the components themselves. Your expensive quad will end up with the equivalent of cancer. Stu
