This appeared in our User Guides a long time ago, but is still relevant information and best practice to protect Ethernet connections against corrosion.
Use dielectric grease (which is uniformly non-conducting) on all connections and in all RJ-45 Ethernet connectors. The best practice is to use enough grease to fill the RJ-45 female connector, and then insert the RJ-45 male connector and push the grease further into the Canopy unit and around the RJ-45 connector. Excess grease can be wiped over the connector area to provide some resistance to water ingress around the connector.
Below are a couple of pictures on how one would apply this compound.
You can use any non-conducting dielectric grease.
Here are links a few choices (these are not explicitly recommended, only some that are readily available):
GC Electronics - Silicone Dielectric Compound
Tekonsha 7200P ElectroTek Non-Conductive Dielectric Grease