Well, somewhere along this ever-lengthening thread, I misspoke. Maybe. It depends on the type of union you have on top of the pump. There is one configuration, the one I have, and the one that Jim was describing, that doesn't need any sealant of any kind, anywhere, because the sealing is done via two o-rings. One is inside the union joint. Only lube for that one. The threads that compress that o-ring don't require any putty or tape. But unbeknownst to me earlier, there is a second o-ring. That o-ring is located between the bottom half of the union and the pump's body. That connection doesn't need sealant either (only lube for the o-ring).
Now where I got off track was your pic, the red arrow. That looks to me to be pointing at putty of some sort, and I can't see if your union has an o-ring where it threads into the pump. It looks like it is just threaded in, which means it's not using an o-ring to seal, rather it's relying on the threads, in which case it would need sealant, and like you, I prefer tape.
So first step is to see if there is an o-ring at that connection, or if it is just threaded in.
