I always order my HB boards with the coating HB offers. I have never added anything else. My temp/hum/solar board sits in the airstream of my FARS bare, ie no box at all, and I have had no problems with it in around two years. I do have the humidity sensor in a "sock" of very fine plastic fabric. It is mounted behind the mounting post with the component side facing downstream.
I use the normal HB cat5 connectors, I spray the sockets and plugs with contact treatment which is a product called CRC 2-26. I make sure I can see the treatment has soaked right into the cat5 plugs (and, therefore, the ends of the wires). It is not a sealant. As long as the cat5 connections are out of "direct" weather, I have found no need for any additional measures.