I used a silicone resin that is used to seal printed circuit boards, applied with a small brush. After making a clean & secure connection, I think you could use any number of liquid coatings that are water proof after drying or curing - perhaps even something like clear nail polish. I honestly have not had an electrical connection issue of any kind in over 23 years with my first homebuilt, which I still fly regularly. That was one of the details I used where I thought it was beneficial on that airplane.