Tarnishing of the brass threads of our bulbs can happen when water gets inside the socket and lingers there. If it's a substantial amount of water, this can cause shorting of the bulb contacts, but more often, it will simply result in tarnishing on the threads. This is not ideal, but it's not dangerous and can easily be removed with a soft rag and some vinegar.
To prevent or minimize tarnishing, we typically suggest having the bulbs point downward so that water isn't able to drip into the sockets. If you really want your bulbs installed pointing up, we offer bulb gaskets as a way of keeping moisture out of the sockets when pointed in an upward fashion.
If you typically install your bulbs pointing upward, you should consider adding them to your outdoor decorating toolkit. They are available in both C7 and C9 sizes.
Please note: We do not recommend using gaskets if you have your bulbs pointed downward, as it can actually have the opposite effect, trapping moisture that may have condensed from the back side of the socket where the wires enter the socket.